Plex: Show watched status for movie and TV requests

[sephrat]

This brings #4881 and #4928 to Plex users.
Feature "PlayedSync" still needs to be manually switched on.

Under the hood:

• Refactored PlexContentSync in a similar way to what was done with EmbyContentSync: Content sync and Played sync share a common base in PlexLibrarySync.
• Refactored users token handling from PlexWatchlistImport to OmbiUserManager so it can be reused in PlexPlayedSync.
• User token error handling is not as robust as it is in PlexWatchlistImport for now. I would vote for renaming PlexWatchlistUserError to something more generic like PlexUserError so we can reuse it in PlexPlayedSync (and/or OmbiUserManager).