speed up account settings users

fixes: CNVS-14742

Preloads admin users communication channels. In role overrides
it changes the class call for context to index by context_id.
Since we know they should always be accounts.

Test Plan:
  - The initial Account settings page should load quicker.
  - Full permission regression testing.

Change-Id: Iadeb55e3b6163421605d1829fa4789c5e1fe9a7e
Reviewed-on: https://gerrit.instructure.com/40360
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Nick Cloward <ncloward@instructure.com>