(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/models/AppModel.ts

@@ -60,6 +60,8 @@ export interface AppModel {
 
   currentOrg: Organization|null;        // null if no access to currentSubdomain
   currentOrgName: string;               // Our best guess for human-friendly name.
+  isPersonal: boolean;                  // Is it a personal site?
+  isTeamSite: boolean;                  // Is it a team site?
   orgError?: OrgError;                  // If currentOrg is null, the error that caused it.
 
   currentFeatures: Features;            // features of the current org's product.
@@ -180,6 +182,9 @@ export class AppModelImpl extends Disposable implements AppModel {
   // Figure out the org name, or blank if details are unavailable.
   public readonly currentOrgName = getOrgNameOrGuest(this.currentOrg, this.currentUser);
 
+  public readonly isPersonal = Boolean(this.currentOrg?.owner);
+  public readonly isTeamSite = Boolean(this.currentOrg) && !this.isPersonal;
+
   public readonly currentFeatures = (this.currentOrg && this.currentOrg.billingAccount) ?
     this.currentOrg.billingAccount.product.features : {};