- Update migration instructions
    - Changes regarding redirect URLs and cookie domain are to permit the
      site to run on multiple domains.
- Set LOGIN_URL in common so that it can be unset in environment overrides

This bypasses the "redirect to LMS" login/signup code, but does not yet
remove it; removal is covered by DEPR-166 so that this remains a
configuration-only change for now.

There should have no user-visible effect.

ref: ARCHBOM-1890