Fix edX name hardcoded in password reset email

Updating commits from #5863 with a bunch of needed fixes, added tests, and squashed commits.

[OSPR-187](https://openedx.atlassian.net/browse/OSPR-187)
Ideally, I'd like to see a unit test to cover the usage of microsite overrides. We have a common/djangoapps/student/tests/test_microsites.py which could provide you with with example code.