Enhancement: Custom WEB App Loading

We've added a new feature which allows the administrator of the environment to provide custom web applications to the
users. This feature is useful for organizations that have specific web applications that they want to provide to their
users.

The users will then be able to access these custom web applications from the web ui.
For a detailed description of the feature, please read the WEB service README.md file.

https://github.com/owncloud/ocis/pull/8523
https://github.com/owncloud/ocis/issues/8392
