Course admin should be able to de-admin dicussion admins

As a global staff or course staff once you added yourself
in the discussion admins you cannot revoke it back unless
you remove yourself as course staff. Any of the course staff
member should be able to remove others from discussion admins
whether they are course staff or not.

TNL-315
We could improve coverage and reduce the amount of code by just using loops:

```
for user in [self.instructor, self.other_user]:
    for identifier_attr in [self.email, self.username]:
        for rolename in ["Administrator", "Moderator", "Community TA"]:
            for action in ["allow", "revoke"]:
                self.assert_update_forum_role_membershipt(user, getattr(user, identifier_attr), rolename, action)
```

Note that the use of getattr here would remove the need to extract the actual identifier from the user object within assert_update_forum_role_membership.