(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/server/lib/sendAppPage.ts

@@ -41,7 +41,7 @@ export function makeGristConfig(homeUrl: string|null, extra: Partial<GristLoadCo
     stripeAPIKey: process.env.STRIPE_PUBLIC_API_KEY,
     googleClientId: process.env.GOOGLE_CLIENT_ID,
     googleDriveScope: process.env.GOOGLE_DRIVE_SCOPE,
-    helpScoutBeaconId: process.env.HELP_SCOUT_BEACON_ID,
+    helpScoutBeaconId: process.env.HELP_SCOUT_BEACON_ID_V2,
     maxUploadSizeImport: (Number(process.env.GRIST_MAX_UPLOAD_IMPORT_MB) * 1024 * 1024) || undefined,
     maxUploadSizeAttachment: (Number(process.env.GRIST_MAX_UPLOAD_ATTACHMENT_MB) * 1024 * 1024) || undefined,
     timestampMs: Date.now(),