Replace `check_for_get_code` with `assert_request_status_code`

More flexible, uses the word "assert", method instead of function
make expanding explanation accessible

LMS-11161
Toggle aria-expanded attribute
Catch InvalidKeyError in UserTagsEventContextMiddleware.

LMS-11189
Catch InvalidKeyError in static_tab view.

LMS-11189
Merge pull request #4678 from edx/usman/lms11189-user-api-middleware-exception-2

Catch InvalidKeyError in UserTagsEventContextMiddleware and static_tab view.
Merge pull request #4654 from edx/db/kill-check-for-code

Replace `check_for_get_code` with `assert_request_status_code`
Merge pull request #4602 from edx/db/expand-on-click-or-enter

make expanding explanation accessible
BLD-1125: Validation of content experiments after managing a group.
Added styling.
Added Jasmine tests.