message sent counter?

[eeeeeta]

People running the bridge may want to keep a track of how many messages they've sent, in order to figure out when they need to top up a PAYG SIM. We should keep a track of that in the DB, and alert the user in the admin room at 90%, 95%, etc. of quota used (where the quota should be user-configurable).
(Also, if we wanted to go full Snapchat, we could adapt this feature into one of those garbage messages sent:received ratio things...)