Summary:
- Add a /welcome/info endpoint, to serve a page after /welcome/user
- Add a new forms module to factor out the styles that feel more natural for a web form.
- Simplify form submission using JSON with a BaseAPI helper.
- The POST submission to /welcome/info gets added to a Grist doc, using a
specialPermit grant to gain access. A failure (e.g. missing doc) is logged
but does not affect the user.
Test Plan: Added a test case.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D2640
