IRCv3: rejoin channels after reconnect

We rejoin after registration to make sure that the user is identified and has
all their user modes set and whatever. We don't actually have a way to do this
right now, but plugins can tie into the signal for that.

Testing Done:
Joined some channels on my local ergo, disconnected the account in the account manager, re-enabled it, and verified via another client that I rejoined those channels.

Reviewed at https://reviews.imfreedom.org/r/2829/