Activation email does not have the correct domain in the activation link

**Describe the bug**

When configuring email settings using the environment variable `FLAGSMITH_DOMAIN` the Activation email (confirming the user registration) does not contain the appropriate domain.

**To Reproduce**

Steps to reproduce the behavior:

1. Set the environment variables for configuring emails.
2. Set the `FLAGSMITH_DOMAIN` environment variable.
3. Set the `ALLOW_REGISTRATION_WITHOUT_INVITE` set to `false` and `ENABLE_EMAIL_ACTIVATION` set to `true`
4. Invite a user to join Flagsmith
5. The user clicks the invite link within the invitation email and is routed to the registration url
6. Register to join Flagsmith and receive the Activation email
7. The email confirmation link does not contain the appropriate domain

**Expected behavior**

The appropriate domain is in the Activation email after the `FLAGSMITH_DOMAIN` environment variable is set

**How are you running Flagsmith?**

- [ ] Self Hosted with Docker
- [X] Self Hosted with Kubernetes
- [ ] SaaS at flagsmith.com
- [ ] Some other way