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

[roborourke]

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