Ticket #2899: clean up inbox/outbox DM form a bit:

Ticket #2899: clean up inbox/outbox DM form a bit:
- "To" drop-down list now defaults to showing "Select recipient:" instead of the first person on your list, reducing liklihood of accidentally sending a message to the wrong person.
- When there are no mutual subscribers to send to, instead of an empty list the list now shows 'No mutual subscribers.'

In both cases, attempting to send when the default is selected displays an error message.
I'm not disabling form elements in part because our themes right now don't show disabled button state correctly; we might want to tighten that up a bit more once fixed.