LTI: include_host for return_url on new_tab launches

test plan:
  * create an LTI assignment with "Launch in a new tab" enabled
  * launch the assignment
  * verify that, in the POST request, the launch_presentation_return_url
      parameter includes protocol and hostname

Change-Id: I54e903cf7916627ed77a2a0cccb4b95da40c6316
Reviewed-on: https://gerrit.instructure.com/43726
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: August Thornton <august@instructure.com>