(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/OpenUserManager.ts

@@ -0,0 +1,21 @@
+import {loadUserManager} from 'app/client/lib/imports';
+import {AppModel} from 'app/client/models/AppModel';
+import {FullUser, Organization, UserAPI} from 'app/common/UserAPI';
+
+// Opens the user-manager for the given org.
+export async function manageTeamUsers(org: Organization, user: FullUser|null, api: UserAPI) {
+  (await loadUserManager()).showUserManagerModal(api, {
+    permissionData: api.getOrgAccess(org.id),
+    activeUser: user,
+    resourceType: 'organization',
+    resourceId: org.id,
+    resource: org,
+  });
+}
+
+// Opens the user-manager for the current org in the given AppModel.
+export async function manageTeamUsersApp(app: AppModel) {
+  if (app.currentOrg) {
+    return manageTeamUsers(app.currentOrg, app.currentValidUser, app.api);
+  }
+}