(core) Automatically or explicitly show document tours in the same way as example cards.

Summary:
Extracts code from showExampleCard into a generic function which is reused for document tours.

It handles reading and writing to user preferences for automatic showing and explicitly reopening.

Test Plan:
Manually tested that it automatically shows a tour just once and clicking to reopen works.

There's not much new functionality so there's little that needs testing. This is an initial version that's mostly internal and is likely to be polished for users in the future.

If I should still add tests, I'd like confirmation that the current behaviour is as desired.

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2944

app/common/Prefs.ts

@@ -23,6 +23,9 @@ export interface UserOrgPrefs extends Prefs {
   // 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[];
+
+  // List of document IDs where the user has seen and dismissed the document tour.
+  seenDocTours?: string[];
 }
 
 export type OrgPrefs = Prefs;