(core) Ask the user some questions after they sign up and set their name.

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

app/common/BaseAPI.ts

@@ -37,6 +37,12 @@ export class BaseAPI {
     };
   }
 
+  // Make a JSON request to the given URL, and read the esponse as JSON. Handles errors, and
+  // counts pending requests in the same way as BaseAPI methods do.
+  public static requestJson(url: string, init: RequestInit = {}): Promise<unknown> {
+    return new BaseAPI().requestJson(url, init);
+  }
+
   private static _numPendingRequests: number = 0;
 
   protected fetch: typeof fetch;