Multiple fixes for test consistency

 - Rewrite ``framedDisconnectionModalShowing`` in
   ``test.session-joining.selenium.js`` to correct its logic. Instead of
   merely checking that the modal's dom element exists, check that it
   exists and is currently showing.  The old logic was incorrect, and
   only passed if the test runner inquired about the dom element during
   an unfinished page load, something that happened more consistently on
   faster machines, and less consistently on constrained environments
   like travis-ci.  This is perhaps the biggest contributor to test
   inconsistency this test suite has had to date.

 - Instead of throwing an alert on "hideApp" in mock-unhangout, just
   log, so that selenium drivers don't have to contend with clearing an
   alert if they take longer than 15 seconds to load.

 - Pass if video embeds in create-hoa to return as either "http:" or
   "https:"; we can't predict which the youtube CDN's will deliver.

 - Tolerate redraws of sessions in the moments before attempting to
   delete them in ``test.session-joining.selenium.js``