Locks a topic.
.. either by way of a moderator or the topic starter.
What this does:
- locks a topic, toggles between locked/unlocked/admin locked.
- only admins can unlock topics locked by other admins.
- requires the lock_own or lock_any permission.
- logs the action to the moderator log.
- returns to the topic after it is done.
- it is accessed via ?action=lock.