Automated posts when a user submits to Thanksbot

[mjchamplin]

Whenever a user submits a thanks, the bot posts in #general prompting other users to do the same. I think this is a good functionality, but with two caveats:

1. Frequency: I think if someone submits several thanks, perhaps this message gets throttled. So if user submits several thanks in quick succession, the bot waits a few minutes then posts a single prompt/update in the general channel.

2. Variety: It would be cool if there was an array of potential messages that were posted. If y'all think that might be a good option, I'd be happy to help write some copy.