models: New PresenceSequence model and UserPresence.last_update_id col.

Migration plan:
1. Add NULLable .last_update_id column to UserPresence with default 0
   for new objects.
2. Backfill the value to 0 for old UserPresences, can be done in the
   background while server is running.
3. Make the column non-NULL.
4. Add new model PresenceSequence and create its rows for old realms.

zerver/lib/server_initialization.py

@@ -13,6 +13,7 @@ from zerver.models import (
     UserProfile,
 )
 from zerver.models.clients import get_client
+from zerver.models.presence import PresenceSequence
 from zerver.models.users import get_system_bot
 from zproject.backends import all_default_backend_names
 
@@ -48,6 +49,8 @@ def create_internal_realm() -> None:
         ]
     )
 
+    PresenceSequence.objects.create(realm=realm, last_update_id=0)
+
     # Create some client objects for common requests.  Not required;
     # just ensures these get low IDs in production, and in development
     # avoids an extra database write for the first HTTP request in