Changed TinyMCE help shortcut key to ALT+F8

Fixes CNVS-1802

Test plan:
- Visit an RCE editor in Canvas.
- Press ALT+F8 to confirm that the help dialog appears.
- Using a screen reader, verify that the user is told that they can press
  ALT+F8 to get help.

Change-Id: I068c6f519df7ebaa94ffb846a82b5d48667954f4
Reviewed-on: https://gerrit.instructure.com/31745
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>