(core) Add Grist forgot password page

Summary:
The page isn't yet linked to from anywhere in the UI, but
will be soon, once the new login page is ready. The page
can still be accessed at login-[s].getgrist.com/forgot-password,
and the flow is similar to the one used by Cognito's hosted UI.

Also refactors much of the existing login app code into smaller
files with less duplication, tweaks password validation to be closer
to Cognito's requirements, and polishes various parts of the UI,
like the verified page CSS, and the form inputs.

Test Plan: Browser, server and project tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3296

stubs/app/server/lib/logins.ts

@@ -7,7 +7,3 @@ export async function getLoginSystem(): Promise<GristLoginSystem> {
   if (saml) { return saml; }
   return getMinimalLoginSystem();
 }
-
-export function getLoginSubdomain(): string | null {
-  return null;
-}