Follow-ups:

* Using a calculation in updateElementsHeight instead of a padding-top for the
  changing the form's height when the button is bigger.
* Add documentation for the new functions
* Made clearer that some functions are internal
* renamed the local `doSendMessage` function to `next`
* extracted in its own function the code that builds a `sendOpts` object that we
  use in both sendSMS and sendMMS.
* sanitize input serviceId in MessageManager's sendSMS and sendMMS.