prevent IRC message flooding

[sknebel]

A slack user was just kicked from Freenode due to posting to much to quickly. Tthey wrote one long slack message, which was split by the bot and resulted in too many messages quickly one after another.

https://chat.indieweb.org/2017-10-18#t1508328890337000 (The actual quit message isn't shown, but immediately before them joining here [heydon] (~slackuser@chat.indieweb.org) has quit (Excess Flood))

The bridge should apply rate limits to prevent this from happening. The matrix-IRC bridge also creates links for very long posts and sends those to IRC instead, a pattern that might be an option here as well (for the IndieWeb channels, an advanced step would be to add the full message to the logs and then link that, but I'm aware this doesn't really fit the architecture).