Add plex endpoints

[revam]

Added new plex endpoints in the v3 api to authenticate the user, select the active server and library, and to sync the user. Also fixed the v2 endpoints so they do what you would expect (e.g. the sync user endpoint would only sync for that one user, etc.)

Changes in this PR

• Fixed the v2 endpoints

• Fixed the sync all endpoint in v3

• Added user sync endpoints

• Added plex auth. and server/library endpoints.

[Cazzar]

"Fixed the v2 endpoints" What was wrong with them?

[revam]

"Fixed the v2 endpoints" What was wrong with them?

The incorrect mapping of the id/uid and the sync for user by id endpoint syncing for all the users instead of just the selected user.

[revam]

A new PR with more internal changes is incoming. Closing this for now.