Update how we get the domain for URLs in templates

[marksmall]

Describe the bug

As noted in https://github.com/astrosat/orbis/issues/754, since the switch to separate client/server containers, the domain in links within the emails are wrong. That issue only mentions password reset, but this affects any email sent e.g. registration confirmation as well. The default was to just get the domain from the current site, but since we now split the client/server, that is no longer the case.

To Reproduce

Steps to reproduce the behavior:

1. Go to orbis
2. Register or reset password
3. See wrong domain in url within email

Expected behavior

I expect the URL in emails to point to the CLIENT container, not the API one.

Screenshots

If applicable, add screenshots to help explain your problem.