Properly handle linked files with direct editing

ToDo

• once done, pick https://github.com/nextcloud/files-clients/issues/13 again

Currently we have no special handling for linking files with direct editing, so this is not working as described in https://github.com/nextcloud/android/issues/7192.

I currently see two ways of implementing support for opening linked files:

• We send a post message to the mobile clients with the path/file id so they can directly navigate to it and open it with direct editing
• We open a browser with the private link of the file. This has less effort on the client side, but will need the user to be logged in in the browser of course.

In addition there is also no support for adding file links through direct editing, since the user is not authenticated via a session. To solve this we'd need to implement something similar to the Collabora insert image functionality where a postMessage is emitted by the text app. Then the mobile clients would show a file picker and pass the selected file to the webview again.

cc @tobiasKaminsky @marinofaggiana What do you think. Does that make sense?

The issue names no implementation files or tests and leaves two approaches for opening linked files unresolved. Start by reviewing nextcloud/files-clients#13 and nextcloud/android#7192, then decide the client and webview integration path; done means linked files can be opened and added through direct editing.