Add public form access behavior and ability to create new user

[kalilsn]

Motivation

• So communities can display links to submission pages that allow non-members to create new pubs
• So only community-level members can use forms to create new Pubs

Requirements

• Updates form permission UI (below) to the checkbox (or toggle, you tell me @ktyjscott!): "Allow non-Members to create new Pubs using this form."
• Updates form rendering behavior based on the permission selected, such that:
  • If checked, if a non-logged-in-user visits the Form link, they are prompted to login or create an account.
  • If checked, and the user is logged in and visits the /fill link, they can see the form and create a Pub. When they submit, they are automatically added as a Contributor member to the Pub.
  • If not checked, and a user visits the /fill link, form checks if user is a Community-level contributor or above member; if they are, they are allowed to see the form. If not, they are given a 404.
  • If not checked, and the user visits a pre-filled link, form checks that the user has access to edit that Pub. If not, they are shown a 404.

Acceptance Criteria

• Migrate form permissions column to a boolean? Or keep it as enum but only allow two choices and migrate the "private" choices to equivalent of "invite-only"?

Mockups (if available)

We already have the dropdown for selecting these, but the data isn't actually used.

[gabestein]

• Should community contributorship do anything?
• Do we want form permissions?
• Who should have access to forms in which cases?
• Need to have form permissions to edit, don't want to give access to default form editor to all contributors
• Change check from access to pub to access to form?