Refactor webconf invitations, move code to Mconf::Invitation

Cleaning up CustomBigbluebuttonRoomsController and Notifier a bit, moving
the code to Mconf::Invitation. Not 100% yet, but a lot better than before.
Now private messages for webconf invitations are being sent again.

Also added a custom mime type :pm to use when defining partials that will be
rendered into private messages.