Bug 887712 - [SMS/MMS] Make editable cursor element "zero width" until any user interaction r=gnarf

https://bugzilla.mozilla.org/show_bug.cgi?id=887712

Includes: https://github.com/gnarf/gaia/commit/4880acb5ecf10b274dd8f6cf5e75b7d9ab5f8ab3

- Empty, out of focus placeholders contract to zero offset width
- Empty, in focus placeholders expand to rem-based width (margin: r & l, padding: r & l)
- When all recipients are deleted, don't leave empty editable in the to-field.
- When the editable placeholder is contracted to a zero offset width, it's possible to accidentally tap a recipient when the intention is to tap the to-field area around the recipient list, which will correctly prompt the user to remove the recipient. Since there is no way to unambiguously detect the user's intention, always handle "Remove" and "Cancel" in an intuitive way.

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