Enhancement: Darkmode theme switcher

We've added a theme switcher and now initialize the user interface theme based on the user's browser preferences.
It also gets saved to the localstorage of the browser so the user's preference gets saved locally.

https://github.com/owncloud/web/issues/6242
https://github.com/owncloud/web/pull/6240
https://github.com/owncloud/web/pull/6350
