add streamStart and forumMention notification handlers

[jonbgamble]

Veloce no need to take action on this. There is a corresponding PR on lila that creates new notification types so the time may come that this (or a subsequent revision) will need to be merged, but that time is not now.

Actually, if you could offer expertise on the best way to launch a forum post in an authenticated browser session, that would be cool. I just threw some calls to showExternalBrowser in there and called it a day. Unauthenticated, but better than nothing.