use selected language for time format, add manual selector

You can now choose between 12h and 24h time in the top left language
menu. Your preference is stored by the browser for future visits.

ts/modules/accounts.ts

@@ -142,6 +142,11 @@ class user implements User {
         };
 
         this.update(user);
+        
+        document.addEventListener("timefmt-change", () => {
+            this.expiry = this.expiry;
+            this.last_active = this.last_active;
+        });
     }
 
     private _updateEmail = () => {