Signups after invite links not working when `PREVENT_SIGNUPS` enabled

### How are you running Flagsmith

- [x] Self Hosted with Docker
- [x] Self Hosted with Kubernetes
- [ ] SaaS at flagsmith.com
- [x] Some other way (add details in description below)

### Describe the bug

When an organisation invite link is opened, the signup page is displayed even if `PREVENT_SIGNUP` is enabled. However, if a user tries to sign up from this page, they receive an error message with no information:

### Steps To Reproduce

1. Deploy the default Docker Compose with `PREVENT_SIGNUP: true`
2. Open an organisation invite link and try to sign up

### Expected behavior

When `PREVENT_SIGNUP` is enabled, users should be allowed to sign up only when they are provisioned by SSO or following an invitation link.

### Screenshots

_No response_