Provide a way to remove a bridged client entirely

[kegsay]

If someone removes their Matrix account (or deactivates or whatever), there should be a way to tell the bridge to stop bridging that connection entirely.

The HS doesn't currently poke ASes when accounts are deactivated (and that wouldn't help for federated HSes) so we can't really do it like that. We can add a !disconnect or !kill option to the IRC admin room, but that doesn't help for the general case where we have guests piling up.

[kegsay]

This is very similar to #74

[lukebarnard1]

We now have !quit (#253) but we still need to deal with guests inactive users piling up.

[kegsay]

Dealing with inactive users piling up isn't an IRC bridge specific problem - we have this issue on Matrix HQ. I'd rather we make a Matrix solution which the bridge piggybacks off on (eg idle timers resulting in kicks, which we already mirror).