Don't populte URL for invalid content item types
Don't populte URL for invalid content item types

Closes PLAT-4485

Test Plan
- Install a 1.3 tool that does dep linking at
  assignment_selection and module items
- Attempt to create an assignment with the tool, but return
  an image rather than an LTI link
- Verify an error is shown and the url field is not populated
- Verify you can create an assignment if an LTI link is
  returned
- Verify the same for module items

Change-Id: I39390bbb570e14625e8ab92d5afa7a85d44376bf
Reviewed-on: https://gerrit.instructure.com/194113
Tested-by: Jenkins
Reviewed-by: Marc Phillips <b810f855174e0bc032432310b0808df2cf714fbc@instructure.com>
QA-Review: Marc Phillips <b810f855174e0bc032432310b0808df2cf714fbc@instructure.com>
Product-Review: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms