Remove the Toast that always says 'Message Sent' and show graphical
indicators instead that show either:
* message is waiting to be sent
* message was sent (or requested)
* message was delivered

The text of the startup failure notification is unhelpful due to lack of
space. Touching the notification now launches an activity that gives details
of the problem and what can be done about it.

Closes #38

We don't need two separate executors for long-running IO threads.