Improve user messaging during initialization

- We first show a localized loading message.
- If initialization takes longer than a certain threshold, we show a different
  ‘optimization’ message.
- If initialization is below the threshold the message change is canceled right
  before the regular loading screen.

background.html

@@ -961,7 +961,7 @@
         <span class='dot'></span>
         <span class='dot'></span>
       </div>
-      <div class='message'>Loading...</div>
+      <div class='message'></div>
     </div>
   </div>
 

js/background.js

@@ -17,6 +17,7 @@
     const { IdleDetector, MessageDataMigrator } = Signal.Workflow;
     const { Errors, Message } = window.Signal.Types;
     const { upgradeMessageSchema } = window.Signal.Migrations;
+    const { Views } = window.Signal;
 
     // Implicitly used in `indexeddb-backbonejs-adapter`:
     // https://github.com/signalapp/Signal-Desktop/blob/4033a9f8137e62ed286170ed5d4941982b1d3a64/components/indexeddb-backbonejs-adapter/backbone-indexeddb.js#L569
@@ -74,6 +75,8 @@
         return accountManager;
     };
 
+    const cancelInitializationMessage = Views.Initialization.setMessage();
+    console.log('Start IndexedDB migrations');
     storage.fetch();
 
 
@@ -163,6 +166,7 @@
             connect(true);
         });
 
+        cancelInitializationMessage();
         var appView = window.owsDesktopApp.appView = new Whisper.AppView({el: $('body')});
 
         Whisper.WallClockListener.init(Whisper.events);