Bug 905950 - [Messages] Recipient assimilation should only occur when composing a new message r=gnarf

https://bugzilla.mozilla.org/show_bug.cgi?id=905950

- Restrict recipient assimilation:
    - Only when in #new view
    - Only when there is a placeholder to inspect

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>