Per-user setting for ui mode (classic/notebookbar)

• Add an option to switch the UI mode per user
• PostMessage for switching ui_defaults on the fly should be also available -> https://github.com/CollaboraOnline/online/issues/3955

Work in progress for adding a user interface switch into online: https://github.com/CollaboraOnline/online/pull/4254

Once that is done and the post messages are properly emitted, we can store the user preference and don't need an extra setting within Nextcloud.

Steps left

• [ ] Catch post message and store it in the user preferences
• [ ] Add option to the user settings
• [ ] Add option to configure already existing app config value from the admin settings
• [ ] Maybe show a nice switcher with some UI screenshot on how the different classic/tabbed layouts look

Hi Are you accepting a PR for this enhancement? I would like to give it a try:)

Sure, feel free to go ahead. Let us know if you have any questions :)