Add availability indicators on people tiles and share fetch service

New Availability module decorates each people-tile with a vertical
  status bar on the avatar and a dot+timestamp under the status text,
  showing ZeitConsens loggedIn state per extension.

  Status and Availability now share a single AvailabilityService that
  polls /availability every 30s, halving the API load. Avatars are
  enlarged to 57px and vertically centered to fit the new layout.

client/src/availability-info.ts

@@ -0,0 +1,5 @@
+export class AvailabilityInfo {
+  public user: string;
+  public loggedIn: boolean;
+  public extension: string;
+}