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
Re-factor Attachment to support audio and video
Begin correcting styling issues
Re-factor attachments to be responsive
Prevent premature revocation of attachment object URLs
Make attachments aware of their origin

This allows for specialized rendering of unsent attachments.
Improve Attachment API

Initialize with an options object, and re-factor "origin" string
attribute to "isDraft" boolean attribute.
Finish implementation
Update unit tests
Fix Attachment initialization
Add conservative delay to Object URL revocation
Ensure image loading element does not trigger overflow
Implement Corey's feedback