Bug 868056 - Clean up SMS test suite r=julien

removes sms_test_html_mock.js which is no longer used since f6667d3a6a8e1954c3df4c12aba462670c36eecf
Bug 865411 - [SMS] Banner required to hold message when user reaches the Max Segments in SMS composer r=borja

* implement the notification according to UX and Visual design
* display this notification if we reach the max length
* disable the send button if over 10 segments, but keep it enabled if we have
  exactly the max length
* rewrote the enableSend function to make it more readable and manageable
* use the new markup loader in thread_ui_test.js, and make it global