Supersedes #3325.

This issue is for the addition of a new argument to the !slowmode set command to have the bot time out slowmodes.

Proposed syntax: !slowmode set [channel] <delay> <duration>

Example use: !slowmode set #python-discussion 10s 30m

After expiration the slowmode should probably restore to whatever the previous value is, that way Moderators can still set a permanent value and have that be preserved (i.e. temporarily have a 10s slowmode before reverting to 2s after 2 hours).

We should persist this in Redis as it is non-critical if this information gets lost but will continue across reboots. This can likely be integrated with the existing scheduler.