Render HTML email notifications
Render HTML email notifications

Fixes CNVS-36094

Test Plan:
* Log into a course as a teacher or admin
* Click 'People' on the left nav
* Click '+ People'
* Make sure 'Email Address' is selected
* Enter a valid email address
* Click 'Next'
* Click the select box by the email you entered.
* Click 'Next'
* Click 'Add Users'
* Check your inbox for the email and verify the content displays
  as formatted HTML and not a raw HTML string

Change-Id: I9e419998b85a30dcb79a4263d9ae79c3403f051d
Reviewed-on: https://gerrit.instructure.com/108026
Tested-by: Jenkins
Reviewed-by: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>
QA-Review: Heath Hales <8ca81b68c7e2592ac614bd07f22fcc735ca55f93@instructure.com>
Product-Review: Dan Minkevitch <2591e5f46f28d303f9dc027d475a5c60d8dea17a@instructure.com>

venturehive/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms