get instead of post to create crocodoc_sessions

This redirects to the session url (like canvadocs) instead of returning
a session url.

refs CNVS-13322
fixes CNVS-13445

Test plan:
  * enable Crocodoc
  * make multiple submissions to the assignment (with the same student)
  * make sure you preview all submissions in the speedgrader (with
    Crocodoc)
  * disable Canvadocs
  * make sure the student can preview the submission attachments in his
    files page (in Crocodoc)

  * also make sure that canvadocs still works (just one preview is fine)

Change-Id: Icbea4b8986039d1405300a09f54a878ba1cc288f
Reviewed-on: https://gerrit.instructure.com/35931
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>