Bug 906748 - [Messages] We don't see the cursor when we remove a "number" recipient r=julienw

https://bugzilla.mozilla.org/show_bug.cgi?id=906748

- If nothing "typed" on keyup, assume isDeletingRecipient
- Updates unrelated test in thread_ui

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