Ember Quiz Stats - DIndex Help Dialog

Adds a tiny ? icon next to discrimination index charts that, when
clicked, pops up a message dialog that reads a friendly helpful message
about the chart.

Closes CNVS-13200

TEST PLAN
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  - create a quiz with MC/TF questions
  - take it by enough students to render the discrimination index chart
  - visit the ember stats page
  - verify that you see a ? icon next to the DI chart, click it:
    - you should see a dialog
    - dialog should read the message specified in the JIRA ticket
    - dialog should contain a link that takes you to the support article
    - dialog should be accessible and closable just like the others

Change-Id: I95db931e65444d90c67a2dfb5d4fe29fdb026bed
Reviewed-on: https://gerrit.instructure.com/35321
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>