refactor(plex): the unit's tables move into plex/models

PlexLibrary, PlexShow, PlexSeason, PlexItem, PlexPerson, PlexCollection, PlexState, PlexLink,
PlexPlaylist and PlexPlaylistItem leave app/models.py for app/plex/models.py; app.tables registers
them. Every import is re-pointed; no schema change.

backend/app/discover/rails.py

@@ -11,7 +11,8 @@ from sqlalchemy.orm import Session
 from app import sysconfig
 from app.discover import cards, queries
 from app.discover.queries import RAIL_LIMIT
-from app.models import PlexShow, User
+from app.models import User
+from app.plex.models import PlexShow
 from app.plex.lookups import by_rating_key
 from app.plex.rollup import NO_TS