(core) Refactor forms implementation

Summary: WIP

Test Plan: Existing tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D4196

buildtools/webpack.config.js

@@ -15,6 +15,7 @@ module.exports = {
     errorPages: "app/client/errorMain",
     apiconsole: "app/client/apiconsole",
     billing: "app/client/billingMain",
+    form: "app/client/formMain",
     // Include client test harness if it is present (it won't be in
     // docker image).
     ...(fs.existsSync("test/client-harness/client.js") ? {