Fixes crashing after following steps:
1. Open https://github.com/SerenityOS/serenity
2. Click on "Pull requests" tab
The problem was `navigable` null pointer dereferencing in
`decode_favicon()`. But navigable is null because the document was
created by `parseFromString()` DOMParser API.
With this change we skip fetching initiated by HTMLLinkElement if
document does not have a browsing context:
- Favicon is not displayed for such documents so no need to fetch.
- Stylesheets fetching won't affect such document because style or
layout does not run for them.
We have to unregister link element stylesheets from the old document's
StyleSheetList when moving them into a new document.
This makes it possible to load GitHub contributor graphs. :^)
