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/tests/test_plex_lang.py

@@ -14,7 +14,8 @@ from app.db import get_db
 from app.main import app
 from app import sysconfig
 from app.plex.cards import display_summary, display_title
-from app.models import (
+from app.models import User
+from app.plex.models import (
     PlexCollection,
     PlexItem,
     PlexLibrary,
@@ -23,7 +24,6 @@ from app.models import (
     PlexSeason,
     PlexShow,
     PlexState,
-    User,
 )