When someone joins, DM anyone who requested them via DM

[snarfed]

#966 added the ability for people to DM a Bridgy Fed account and request that someone join the bridge. When that person joins, we should DM the original requester to notify them.

Thanks for the nudge @mackuba!

[snarfed]

I may scope this down to just the original requester, or limit to the first N requesters, etc.

[qazmlp]

First N would be fine I think, but please let people know in the initial response to their request if the requestee isn't being messaged again and also if they won't receive a notification.