(core) Adding GristConnect login system

Summary:
New login system to allow simple SSO flow that is based on Discourse description that is available at:
https://meta.discourse.org/t/discourseconnect-official-single-sign-on-for-discourse-sso/13045

Test Plan: New core test.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3418

test/nbrowser/gristUtils.ts

@@ -91,6 +91,13 @@ export function exactMatch(value: string): RegExp {
   return new RegExp(`^${escapeRegExp(value)}$`);
 }
 
+/**
+ * Helper function that creates a regular expression to match the begging of the string.
+ */
+export function startsWith(value: string): RegExp {
+  return new RegExp(`^${escapeRegExp(value)}`);
+}
+
 /**
  * Helper to scroll an element into view.
  */
@@ -2114,7 +2121,7 @@ export function addSamplesForSuite() {
   });
 }
 
-async function openAccountMenu() {
+export async function openAccountMenu() {
   await driver.findWait('.test-dm-account', 1000).click();
   // Since the AccountWidget loads orgs and the user data asynchronously, the menu
   // can expand itself causing the click to land on a wrong button.