ACL Clean command

[ShadowJonathan]

Is your feature request related to a problem? Please describe.
Currently, when applying ACLs, users from the corresponding ACL'd rooms are not automatically "cleaned up" (kicked), this is fine for purposes of temporarily dampening attacks against a server.

However, in the long term, users might still linger in a room, and other than a manual kick or a room upgrade, these users would stay around.

Describe the solution you'd like
A command that clears (kicks) users from a room, based on if they match an ACL. This can be applied on a protected-rooms level, or a specific room level.

!mjolnir acl-clean <room>

[ShadowJonathan]

#273 and #161 are related to this, likely a dup, though this issue explains an approach on how to handle #161.