Allow users to create or login to accounts to fill out forms set to public

[gabestein]

Motivation

So communities can accept e.g. submissions from the public (with minimal spam)

For example, ICOR will want a public submissions form where anyone can submit a project.

Requirements

• When a form is set to "public" in the ui...
  • Visiting the form page while logged in as any user will load it
  • Visiting the form page not logged in will prompt the user to either login or create an account
• Upon submitting the form, if the user is not already a member of the community, they will be given a Contributor membership to the community and a Contributor membership to the resulting pub

Acceptance Criteria

• Enable public signup if not already built -- but only in context of filling out a form, not on other pages (for now)

Extra work

• Change private/invite-only/public dropdown to private <-> public switch. Context here is we are ultimately going to get rid of the concept of "invite-only" forms, and the only setting that matters is whether anyone with an account can fill out a form. But we don't need to make the UI change right now.

[gabestein]

Signup form is at /c/<communitySlug>/signup