feat: remove signup link from landing page

We are deprecating webapp based signup. It leads to an inconsistent state for mail providers and offers no useful interactions for vpn providers either.

Instead of trying to deal with the halve way signup through the webapp we require signup through bitmask app which can also create the pgp keys for email and download and use the cert for vpn.

In addition this reduces the attack surface for js injection, phishing and other browser based attacks.

For now we still keep the signup form in case providers link to it directly. We also keep all the tests based on it. Cleanup will happen right after 0.10.0 release.