Add TinyMCE Keyboard shortcuts dialog for assessments pages

This adds a little 'i' icon that shows the TinyMCE keyboard shortcuts.
It's a critical change to benchmark leading-edge accessibility initiatives

Fixes CNVS-15962

Test Plan:

    Make sure the 'i' icon works on these three pages.  When clicked it
    should display a dialog that shows the keyboard shortcuts for the
    TinyMCE editor:

        1. Submit assignment
        2. Create/Edit assignment
        3. Outcomes

Change-Id: If20bb1a2ffca89417666968f7d6beff0ebe3cbd2
Reviewed-on: https://gerrit.instructure.com/44024
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Adam Stone <astone@instructure.com>
Product-Review: Benjamin Porter <bporter@instructure.com>