(core) Add new Grist sign-up page

Summary:
Available at login.getgrist.com/signup, the new sign-up page
includes similar options available on the hosted Cognito sign-up
page, such as support for registering with Google. All previous
redirects to Cognito for sign-up should now redirect to the new
Grist sign-up page.

Login is still handled with the hosted Cognito login page, and there
is a link to go there from the new sign-up page.

Test Plan: Browser, project and server tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3249

app/server/lib/extractOrg.ts

@@ -131,6 +131,27 @@ export class Hosts {
     return this._redirectHost.bind(this);
   }
 
+  /**
+   * Returns true if `url` either points to a native Grist host (e.g. foo.getgrist.com),
+   * or a custom host for an existing Grist org.
+   */
+  public async isSafeRedirectUrl(url: string): Promise<boolean> {
+    const {host, protocol} = new URL(url);
+    if (!['http:', 'https:'].includes(protocol)) { return false; }
+
+    switch (this._getHostType(host)) {
+      case 'native': { return true; }
+      case 'custom': {
+        const org = await mapGetOrSet(this._host2org, host, async () => {
+          const o = await this._dbManager.connection.manager.findOne(Organization, {host});
+          return o?.domain ?? undefined;
+        });
+        return org !== undefined;
+      }
+      default: { return false; }
+    }
+  }
+
   public close() {
     this._host2org.clear();
     this._org2host.clear();