fix(load): fix window load timing

Summary:
The app regressed to launching a blank white page for a while before
showing the UI. This was because I forgot to await for the now async
package loading

Test Plan: Manually boot app with compile cache disabled

Reviewers: mark, spang, juan

Reviewed By: juan

Differential Revision: https://phab.nylas.com/D3883

src/nylas-env.coffee

@@ -683,9 +683,9 @@ class NylasEnvConstructor
           window.requestAnimationFrame =>
             window.requestAnimationFrame =>
               @displayWindow() unless initializeInBackground
-              @startWindow()
+              @startWindow().then =>
-              # These don't need to wait for the window's stores and such
+                # These don't need to wait for the window's stores and
-              # to fully activate:
+                # such to fully activate:
                 @requireUserInitScript() unless safeMode
                 @showMainWindow()
                 ipcRenderer.send('window-command', 'window:loaded')