add support for submitting URL content via LTI

Test Plan:
 - Create a homework assignment that accepts a website url
 - Add an external tool that supports homework submission
 - Submit a URL from the tool in a tab (not the more tab)
 - It should save the submission

Regression Test
 - File submissions via LTI should still work

fixes PLAT-579

Change-Id: I1a0ef11c0c4293d175c1e7604622922eadb8bf7f
Reviewed-on: https://gerrit.instructure.com/38707
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
Product-Review: Brad Humphrey <brad@instructure.com>