Adding a new signup via network admin results in email with no activation key

Steps to reproduce:

- Go to network admin > signups
- Add new sign up, only enter the email address and intended user name
- Check mailhog and see the activation email is missing the activation key in the link

I suspect this is caching related as we have had similar issues with this plugin in the past.

Acceptance criteria:

- [ ] Sign up activation email link always has an activation key value in the URL