Add periodic messages

[mikeshultz]

A channel admin should be able to set a period message to be sent every N minutes.

/periodic [N minutes] My message

Discontinue

/periodicstop

A secondary request for this was adding CTA buttons. This may need to be split off into a separate issue.

[mikeshultz]

This is pretty straight forward for simple messages, but adding the CTA is a little... weird. It would probably need to be a secondary command entirely. That would require something like this to assemble the whole message:

/periodic 60 OUSD is cool and you should check it out!
/periodic_cta https://ousd.com Buy OUSD!

This should produce a message that is sent every hour that looks something like this:

OUSD is cool and you should check it out!

[Buy OUSD!]

Also, it should be noted that the way the bot is built, it will send out periodic messages only if there's activity in the channel. So if there's no chat activity in an hour, the bot will not send out a message until someone else sends one that triggers the check. This might be able to work with some kind of ioloop but will require more work.