Bug 875564 - [MMS] after attaching image, sometimes send button is still disabled - r=gnarf

- Assimilate recipients when input gains focus
- Assimilate recipients when attachment button is pressed
- Assimilate recipients when send button is pressed

https://bugzilla.mozilla.org/show_bug.cgi?id=875564

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>

Closes gh-10224
(cherry picked from commit fdc0e5ea5461a9464ebf707fd0dcb27a4354a23f)