Fixes#813: Application services were unable to work because their sender_localpart
user was never created in the database, preventing authentication.
This fix ensures the appservice user account is created when:
- The server starts up and loads existing appservices from the database
- A new appservice is registered via the admin command
Additionally, if an appservice user has been accidentally deactivated, it will be
automatically reactivated when the appservice starts.
The solution centralises all appservice startup logic into a single `start_appservice`
helper method, eliminating code duplication between the registration and startup paths.
e820551f62