(core) Update HelpScout beacon to work with embedded documentation articles.

Summary:
- Fix base href in HelpScout beacon when showing articles (in particular for Firefox)
- Show the 'Answers' tab normally except when reporting an error.
- Combine the "Give Feedback" and "Help Center" buttons into one that normally
  opens the beacon (with a link to Help Center and to Community Forum), and a
  smaller one that opens the Help Center site in a new tab.
- Update HELP_SCOUT_* env vars to use _V2 suffix, to allow them to coexist with
  code using the previous beacon.

Test Plan: Updated the browser test to check the new behavior.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3170

app/gen-server/ApiServer.ts

@@ -39,7 +39,7 @@ export function getOrgFromRequest(req: Request): string|null {
  * HelpScout the user identity for identifying customer information and conversation history.
  */
 function helpScoutSign(email: string): string|undefined {
-  const secretKey = process.env.HELP_SCOUT_SECRET_KEY;
+  const secretKey = process.env.HELP_SCOUT_SECRET_KEY_V2;
   if (!secretKey) { return undefined; }
   return crypto.createHmac('sha256', secretKey).update(email).digest('hex');
 }