Bug 877615 - [MMS] New message composer: Recipients field stays expanded after discarding a previous multi recipient message - r=gnarf37

- #messages-recipients-list is declared "singleline" in markup
- Test to enforce recipient list always begins in singleline mode

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

Closes gh-10193
(cherry picked from commit 59ee4a159e2e313b736de4ea10e30049d4d93d40)