Multiple account handling

Along with Jellyfin integration, multiple account management is intended to be added, so that you can log in to multiple accounts across both Plex and Jellyfin at once, with easy switching in the user menu.

One of the bigger complexities here is that user preferences are currently linked to your Plex account ID. A new system will need to be created where preferences can be linked to multiple accounts, and remain consistent even if you log in/out of additional accounts.