Put logistration page behind a feature flag

This will make it slightly safer to merge the feature branch into master.  I suggest we do that relatively soon to avoid getting into rebase/merge hell.

To enable the feature flag, set this in `lms.envs.json` under `FEATURES`
```
"ENABLE_COMBINED_LOGIN_REGISTRATION": true,
```

@dianakhuang @AlasdairSwan