(core) Update design of empty docs home page, and add a "Manage Team" button.

Summary:
- Remove the empty-folder icon
- Add an "Invite team members" button for owners on empty team sites
- Add a "Browse Templates" button for all other cases on empty sites
- Update intro text for team, including a link to Sprouts
- Update intro text for personal/anon.
- Include a Free/Pro tag for team sites (for now, only "Free")
- Add a "Manage Team" button for owners on home page of all team sites.
- Polished the UI of UserManager: add a transition for the background, and
  delay the appearance of the spinner for fast loads.

Test Plan: Fixed up the HomeIntro tests; added test case for Manage Team button

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3459

app/client/ui/DocMenu.ts

@@ -67,7 +67,9 @@ function createLoadedDocMenu(home: HomeModel) {
 
         return [
           // Hide the sort option only when showing intro.
-          buildPrefs(viewSettings, {hideSort: showIntro}),
+          ((showIntro && page === 'all') ? null :
+            buildPrefs(viewSettings, {hideSort: showIntro})
+          ),
 
           // Build the pinned docs dom. Builds nothing if the selectedOrg is unloaded or
           dom.maybe((use) => use(home.currentWSPinnedDocs).length > 0, () => [