prevent tinymce from messing with inserted urls
prevent tinymce from messing with inserted urls

fixes CNVS-30495

From what I could tell, all of the link manipulation necessary to make
them work in a variety of different situations (when content is render
on the web; when content is exported / importer; when content is
provided as part of an API response) is handled manually by code in
canvas. By explicitly telling tinymce not to attempt to manipulate
inserted link urls at all, we solve this one case where it was actually
getting in the way: linking to absolute urls without including a
trailing slash.

test plan:
- Add an assignment with relative links to content within the course;
  absolute links to content within the course; absolute links to a
  domain with and without trailing slash.
- View the assigment in the browser. Observe that all links work.
- Fetch the assignment from the API and observe that all links are
  absolute.
- Export the course and import into a new one. Observe that all links
  work.

Change-Id: I7dbb2e1a0462ae641b68453887add0a5fcda21e6
Reviewed-on: https://gerrit.instructure.com/86942
Reviewed-by: Brent Burgoyne <bccf16d7abdd2611580b1cd7d7a6fcc150d619da@instructure.com>
Tested-by: Jenkins
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
QA-Review: Benjamin Christian Nelson <4a283f6b032925e40a485c88ede5597d7d65a090@instructure.com>
Product-Review: John Corrigan <3d73713d0cc718326e4fd7c06c64e067ad6e2270@instructure.com>

instructure/canvas-lms,venturehive/canvas-lms,dgynn/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,SwinburneOnline/canvas-lms,dgynn/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,HotChalk/canvas-lms,instructure/canvas-lms,dgynn/canvas-lms,djbender/canvas-lms,matematikk-mooc/canvas-lms,matematikk-mooc/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,HotChalk/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,instructure/canvas-lms,dgynn/canvas-lms,instructure/canvas-lms,HotChalk/canvas-lms,matematikk-mooc/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,HotChalk/canvas-lms