(core) Show a welcome card when a user opens an example for the first time.

Summary:
- The card includes an image, a brief description, and a link to the tutorial.
- The left panel includes a link to the tutorial, and a button to reopen card.
- Card is collapsed and expanded with a little animation.
- Add a seenExamples pref for whether an example has been seen.
- Store the pref in localStorage for anon user.

Separately, added clearing of prefs of test users between tests, to avoid tests
affecting unrelated tests.

Test Plan: Added a browser test.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2602

app/common/Prefs.ts

@@ -18,6 +18,11 @@ export type UserPrefs = Prefs;
 export interface UserOrgPrefs extends Prefs {
   docMenuSort?: SortPref;
   docMenuView?: ViewPref;
+
+  // List of example docs that the user has seen and dismissed the welcome card for.
+  // The numbers are the `id` from IExampleInfo in app/client/ui/ExampleInfo.
+  // By living in UserOrgPrefs, this applies only to the examples-containing org.
+  seenExamples?: number[];
 }
 
 export type OrgPrefs = Prefs;