Upgrade mediaelement.js to 2.15.0

Overall this is a relatively small release, but one with a few notable
changes for Canvas:

 - improvements to keyboard accessibility
 - playback speed support
 - video controls won't hide when they are being hovered over
 - improved play button image
 - proper source chooser support in flash fallback player

The full changelog is here:

https://github.com/johndyer/mediaelement/blob/3be710cc494accd9814b95392461d768ead60f0d/changelog.md

Test Plan:

We'll want to do a basic cross-browser regression test of the media
player to cover the upgrade.

 - On an account with "prefer HTML5 video" enabled:
   - Check a video player
   - Check an audio player
 - On an account with "prefer HTML5 video" disabled:
   - Check a video player
   - Check an audio player

Change-Id: I363e301506981185bb1a37fdd8975ad3e91d5180
[a11y] Quizzes RCE Keyboard Dialog

This patch adds a keyboard-accessible dialog that lists the available
TinyMCE keyboard shortcuts within quizzes.

Closes CNVS-9795

TEST PLAN
---- ----

  - go to the create or edit quiz page
    + verify you see a little "?" help icon next to the "HTML Editor"
      link in the editor action-bar
    + click it and verify that it launches a dialog
      - verify the dialog contents match those specified in the ticket
    + press ALT+0 and verify the dialog gets opened
      - you can try using the 0 on the numpad if you have one, it should
        work too
    + verify that you can get to the "?" icon using your keyboard by
      TABbing around

Change-Id: I18df7ce74d33369df2245e2fff4d9d0c7e1735a2
Adjusts external apps to account for long app names

Converts external_tools.sass -> scss while adding a few styles to
truncate the names properly.  The styles added convert the table
to render using the fixed style and the first column to have a width
of 500px and overflow the text using ellipsis.  A title attribute is
also added to the column to help with UX.

fixes CNVS-7223

test plan:
	- Go to Settings > Apps
	- Add an app with a long name around 160 characters
	- You should see that the name is truncated with an ellipsis
	- Hover over the name
	- A tooltip with the full name should appear

Change-Id: I24cc71a0073799e66efa57421b80b0b70bdbecdb
Accessibility: Caused screen readers to recognize cancel as a button.

Fixes CNVS-14126

Test plan:
- Visit  the Calendar
- Click Add New Event
- Click More Options
- Navigate to the cancel button
- Verify that it is recognized as a button by screen readers

Change-Id: Ice93078d7d728503bc4bda29a5894ae19583692c
add UI option to remove dates in content migration

also remove some code that accomplishes nothing but confusion

test plan: perform content migrations into a course via the web UI
  1. without adjusting dates
  2. shifting dates
  3. removing dates
and verify that the correct operation is performed for each migration.

test all of these workflows:
  1. Course Settings / Copy this Course
  2. Import Content / Copy a Canvas Course
  3. Import Content / course export package

fixes CNVS-14183

Change-Id: I965a8ba6e6b0971cd0b5561627716989191a9680
fallback to entered url if no tool is linked

fixes PLAT-544

test-plan
*manually add a external tool module item by url
*add a external tool that matches the urls domain
*try to launch the toool via the module item
*it shouldn't give you an error

Change-Id: Ie4d017db62a386ca49e4632c94d14261a3960149
accessibility: improved conversations filters

Fixes CNVS-14505

Test plan:
- Visit the inbox
- Using a screen reader, verify that the currently selected item is read as
  the label of the button to open the message filter drop-downs
- Verify that screen reader users can navigateand select items from both lists
- Verify that the appearance remains unaltered.

Change-Id: I1e177ca0be0d6b0c0ad6094de477938dfd440868
Adds type to cancel button to prevent default submit.

fixes CNVS-9975

test plan:
	- Go to a user's profile
	- Click edit
	- Change the user's name
	- Click cancel
	- The user's name should not update.

Change-Id: Iaa2bbeada63eaeb5521bbb1be627f530d6093563
Reviewed-on: https://gerrit.instructure.com/38758
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
change tool proxy root_account to context

also, clean up ims-lti usage with pluralized lti methods

test plan:
  - registrations should still work
  - registrations without resource handlers
    and using a default message handler should still work

fixes PLAT-566 PLAT-567

Change-Id: Icd9feca44ba9f5600d89a4943b0b93b34aeb78a5
Reviewed-on: https://gerrit.instructure.com/38182
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
Product-Review: Brad Humphrey <brad@instructure.com>
QA-Review: Brad Humphrey <brad@instructure.com>