(core) Fix bugs when both welcomeTour and docTour are available

Summary:
- Unify where in the code tours get initiated.
- Avoid start a new tour while one is being started or is in progress.
- Ignore welcome tour when on a doc that has a doc tour.
- Fix tours when starting with a special page like Access Rules.
- Remove mention of the no-longer-present "Give Feedback" button in the last
  message of the welcome tour.

Test Plan:
Add a browser test case that docTour preempts the welcome tour and shows no errors
(this test case fails in multiple ways without the changes).

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3330

app/client/ui/OnBoardingPopups.ts

@@ -80,6 +80,11 @@ export function startOnBoarding(messages: IOnBoardingMsg[], onFinishCB: () => vo
   ctl.start().catch(reportError);
 }
 
+// Returns whether some tour is currently active.
+export function isTourActive(): boolean {
+  return !tourSingleton.isEmpty();
+}
+
 class OnBoardingError extends Error {
   public name = 'OnBoardingError';
   constructor(message: string) {