feat: Custom registration page override.

This MR allows the registration page to be replaced with an offsite URL if desired.

Supporting information

• Jira ticket
• GitLab issue

Merge deadline

"None" if there's no rush, "ASAP" if it's critical, or provide a specific date if there is one.

Testing instructions

Step by step procedure

1. make to get yourself a copy of Listaflow.
2. Edit frontend/.env.local and add the REACT_APP_REGISTRATION_URL environment variable, overwriting with a preferred URL.
3. make stop && make up
4. Visit https://localhost:3000/ and click the 'Sign up' button in the upper right. Verify it goes to the override.
5. Visit the login page. Check the sign up link there and verify it goes to the right location.
6. Unset the variable.
7. make stop && make up
8. Verify the links show the normal reg page again.

Author notes and concerns

None or note it here