Added automod unarchive_thread action

[metal0]

Depends on #292

Adds lock property to archive_thread action to optionally moderator-archive a thread.

The unlock property on the unarchive_thread action is something I'm not sure is gonna be used too much in practice, though it does serve a different functional purpose to just forcefully unlocking/unarchiving threads.
But change it to your liking /shrug

[metal0]

@cla-bot check

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[cla-bot]

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