Change: Theme handling

The handling of themes has been throughoutly reworked.
Themes now feature a human-readable title, can specify whether they are suitable as light or dark mode themes.
If only one theme is provided, this theme will be set.
If two themes are provided and one is a dark and one is a light mode one, there is a light/dark mode switch button in the AppBar.
If more than two themes are provided or the two provided themes are not a dark and a light mode one respectively,
the UI features a theme-switch button featuring a dropdown of the human-readable names of the available themes

https://github.com/owncloud/web/issues/2404
https://github.com/owncloud/web/issues/8424
https://github.com/owncloud/web/issues/9403
https://github.com/owncloud/web/issues/9885
https://github.com/owncloud/web/issues/9939
https://github.com/owncloud/web/issues/10146
https://github.com/owncloud/web/pull/8855
https://github.com/owncloud/web/pull/9396
https://github.com/owncloud/web/pull/9401
https://github.com/owncloud/web/pull/9698
https://github.com/owncloud/web/pull/10151
