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

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

- Set zero-width on editable placeholder until any interaction
- Remove zero-width when no recipients
- Remove zero-width when visibility changes to multi-line
- Remove zero-width during interactions
- Restore zero-width on editable after interaction (signified by loss of focus, <enter> or ";")

This also fixes a small bug in Threads.currentId where currentId can potentially be NaN. The bug was causing test failures for the Threads.currentId API

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