(core) Implement updated DocMenu UI: list/card mode and sort mode.

Summary:
- Add org-wide currentSort and currentView, saved as user preferences.
- Add per-workspace currentSort and currentView, backed by localStorage.
- Move localStorage-based observables to a separate file.
- Move hard-coded data about example docs to a separate file.
- Add UI for toggling sort and view mode.
- Removed unused features of buttonSelect to simplify it,
  and added support for light style of buttons.
- Added `parse` helper method to StringUnion, and use it in a few places where
  it simplifies code.
- Set `needRealOrg: true` in HomeDBManager.updateOrg() to fix saving prefs for
  mergedOrg.

Test Plan: WIP: Fixed some affected tests. New tests not yet written.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2587

app/common/Prefs.ts

@@ -1,3 +1,12 @@
+import {StringUnion} from 'app/common/StringUnion';
+
+export const SortPref = StringUnion("name", "date");
+export type SortPref = typeof SortPref.type;
+
+export const ViewPref = StringUnion("list", "icons");
+export type ViewPref = typeof ViewPref.type;
+
+
 // A collection of preferences related to a user or org (or combination).
 export interface Prefs {
   // TODO replace this with real preferences.
@@ -5,5 +14,10 @@ export interface Prefs {
 }
 
 export type UserPrefs = Prefs;
-export type UserOrgPrefs = Prefs;
+
+export interface UserOrgPrefs extends Prefs {
+  docMenuSort?: SortPref;
+  docMenuView?: ViewPref;
+}
+
 export type OrgPrefs = Prefs;