web: Replace calls to rootInterface()?.tenant? with a contextual this.tenant object (#7778)

* This commit abstracts access to the object `rootInterface()?.config?` into a single accessor,
`authentikConfig`, that can be mixed into any AKElement object that requires access to it.

Since access to `rootInterface()?.config?` is _universally_ used for a single (and repetitive)
boolean check, a separate accessor has been provided that converts all calls of the form:

``` javascript
rootInterface()?.config?.capabilities.includes(CapabilitiesEnum.CanImpersonate)
```

into:

``` javascript
this.can(CapabilitiesEnum.CanImpersonate)
```

It does this via a Mixin, `WithCapabilitiesConfig`, which understands that these calls only make
sense in the context of a running, fully configured authentik instance, and that their purpose is to
inform authentik components of a user’s capabilities. The latter is why I don’t feel uncomfortable
turning a function call into a method; we should make it explicit that this is a relationship
between components.

The mixin has a single single field, `[WCC.capabilitiesConfig]`, where its association with the
upper-level configuration is made. If that syntax looks peculiar to you, good! I’ve used an explict
unique symbol as the field name; it is inaccessable an innumerable in the object list. The debugger
shows it only as:

    Symbol(): {
        cacheTimeout: 300
        cacheTimeoutFlows: 300
        cacheTimeoutPolicies: 300
        cacheTimeoutReputation: 300
        capabilities: (5) ['can_save_media', 'can_geo_ip', 'can_impersonate', 'can_debug', 'is_enterprise']
    }

Since you can’t reference it by identity, you can’t write to it. Until every browser supports actual
private fields, this is the best we can do; it does guarantee that field name collisions are
impossible, which is a win.

The mixin takes a second optional boolean; setting this to true will cause any web component using
the mixin to automatically schedule a re-render if the capabilities list changes.

The mixin is also generic; despite the "...into a Lit-Context" in the title, the internals of the
Mixin can be replaced with anything so long as the signature of `.can()` is preserved.

Because this work builds off the work I did to give the Sidebar access to the configuration without
ad-hoc retrieval or prop-drilling, it wasn’t necessary to create a new context for it. That will be
necessary for the following:

TODO:

``` javascript
rootInterface()?.uiConfig;
rootInterface()?.tenant;
me();
```

* This commit abstracts access to the object `rootInterface()?.tenant?` into a single accessor,
`tenant`, that can be mixed into any AKElement object that requires access to it.

Like `WithCapabilitiesConfig` and `WithAuthentikConfig`, this one is named `WithTenantConfig`.

TODO:

``` javascript
rootInterface()?.uiConfig;
me();
```

* web: Added a README with a description of the applications' "mental model," essentially an architectural description.

* web: prettier did a thing

* web: prettier had opinions about the README

* web: Jens requested that subscription be  by default, and it's the right call.

* web: Jens requested that the default subscription state for contexts be , and it's the right call.

* web: prettier having opinions after merging with dependent branch

* web: prettier still having opinions.

web/src/user/user-settings/details/UserSettingsFlowExecutor.ts

@@ -2,14 +2,15 @@ import { DEFAULT_CONFIG } from "@goauthentik/common/api/config";
 import { EVENT_REFRESH } from "@goauthentik/common/constants";
 import { MessageLevel } from "@goauthentik/common/messages";
 import { refreshMe } from "@goauthentik/common/users";
-import { AKElement, rootInterface } from "@goauthentik/elements/Base";
+import { AKElement } from "@goauthentik/elements/Base";
+import { WithTenantConfig } from "@goauthentik/elements/Interface/tenantProvider";
 import { showMessage } from "@goauthentik/elements/messages/MessageContainer";
 import { StageHost } from "@goauthentik/flow/stages/base";
 import "@goauthentik/user/user-settings/details/stages/prompt/PromptStage";
 
 import { msg } from "@lit/localize";
 import { CSSResult, TemplateResult, html } from "lit";
-import { customElement, property, state } from "lit/decorators.js";
+import { customElement, property } from "lit/decorators.js";
 import { unsafeHTML } from "lit/directives/unsafe-html.js";
 
 import PFButton from "@patternfly/patternfly/components/Button/button.css";
@@ -21,7 +22,6 @@ import PFBase from "@patternfly/patternfly/patternfly-base.css";
 import {
     ChallengeChoices,
     ChallengeTypes,
-    CurrentTenant,
     FlowChallengeResponseRequest,
     FlowErrorChallenge,
     FlowsApi,
@@ -31,13 +31,13 @@ import {
 } from "@goauthentik/api";
 
 @customElement("ak-user-settings-flow-executor")
-export class UserSettingsFlowExecutor extends AKElement implements StageHost {
+export class UserSettingsFlowExecutor
+    extends WithTenantConfig(AKElement, true)
+    implements StageHost
+{
     @property()
     flowSlug?: string;
 
-    @state()
-    tenant?: CurrentTenant;
-
     private _challenge?: ChallengeTypes;
 
     @property({ attribute: false })
@@ -87,7 +87,6 @@ export class UserSettingsFlowExecutor extends AKElement implements StageHost {
     }
 
     firstUpdated(): void {
-        this.tenant = rootInterface()?.tenant;
         this.flowSlug = this.tenant?.flowUserSettings;
         if (!this.flowSlug) {
             return;