make discussion topic editing accessible
make discussion topic editing accessible

closes CNVS-19327
refs CNVS-15543

This applies a screenreader-only label to the date picker used to
edit discussion topics which gives formatting
information, and also adds an on-focus tooltip for KB-only users that gives format
information (the same info for both).

TEST PLAN:
- go to any course and create or edit a discussion topic
- Using a screenreader with your eyes closed, ensure you can figure out
  what
  date format to submit for available boundaries with no additional context.
- tab through to the same fields and prove that you see
  a little tooltip telling you what format to enter the date and time
in.

Change-Id: Ib0396247f7a2d53781e480109aa47d80d93fa955
Reviewed-on: https://gerrit.instructure.com/56634
Reviewed-by: Jason Madsen <bcb5fff571a20f07c35d17ba9c364855c776dce7@instructure.com>
Tested-by: Jenkins
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>
Product-Review: Ethan Vizitei <73eeb0b2f65d05a906adf3b21ee1f9f5a2aa3c1c@instructure.com>

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