This PR enables setting/changing a user's password from the django admin, gated
behind a waffle switch.
*Sandbox URL*: TBD - sandbox is being provisioned.
*Merge deadline*: None
1. login to the admin on the *lms* as a superuser
2. navigate the admin change page for a user (eg `/admin/auth/user/1/change/`)
3. verify that there is no link to a password change form in the description
for the password field. (You should not see " but you can change the
password using this form.")
4. change the current url from .../change/ to .../password/ (eg.
5. verify that a 404 page is displayed
6. repeat steps 2 - 5 on the *cms*
7. navigate to the waffle switch admin page (`/admin/waffle/switch`) - doesn't
matter if cms or lms admin.
8. add a new waffle switch named `common.changeuserpassword_admin` and enable
9. restart the cms and lms apps (reboot the server or run
`/edx/bin/supervisorctl restart lms: cms:` from a shell).
10. log in to the admin on the *lms* as a superuser
11. navigate the admin change page for a user (eg `/admin/auth/user/1/change/`)
12. verify that a link to a password change form ("but you can change the
password using this form") is visible.
13. click the form link and verify that the page loads correctly and the user
password can be changed
14. repeat the previous 4 steps from the *cms* admin.
*Author notes and concerns*:
1. A waffle switch may not be the best approach, because changes aren't fully
applied until the server is restarted. An option under settings might be
more appropriate, as then it will not be possible for the behaviour to be
[ ] @pomegranited
[ ] edX reviewer[s] TBD