Bug 920546 - [Messages] Short-term fixes for the recipient panel

https://bugzilla.mozilla.org/show_bug.cgi?id=920546

- Decouple searchContact from list display
- Create ThreadUI.listContact(...)
- Create ThreadUI.validateContact(...)
- Adds two level validation:
    - Is the entry questionable?
        - This kicks off a silent search for matching contacts
    - Is the entry invalid?
        - This state is reached if no contacts could be found
          for the typed value.
        - Prevents enableSend
        - Prevents "export" of recipient entry at Send
          (when mixed with multiple valid recipients)
- Refactor Utils.getContactDisplayInfo logic
- Adds tests for enableSend

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>