Restructure app init; fixes #5 and #11

Squashed commit of the following:

commit d4242bf1a390994606b7bd6e630c55338a14aad4
Author: Arne Schlüter <arne@schlueter.is>
Date:   Mon Jul 9 21:12:44 2018 +0200

    Add loading spinner, done with reworked app boot flow; fixes #5 and #11

commit e864ae4e578f96b86f3c0239b79f5224f0bb0020
Author: Arne Schlüter <arne@schlueter.is>
Date:   Mon Jul 9 19:43:02 2018 +0200

    Start restructuring app boot flow

commit a8cdbef80acde9f185a588ab86f8ea6964ebe8ab
Author: Arne Schlüter <arne@schlueter.is>
Date:   Mon Jul 9 14:03:43 2018 +0200

    Ignore rebel readline artifacts

commit 67eae3bc6aa2938ad6748c78b6259e532e66f865
Author: Arne Schlüter <arne@schlueter.is>
Date:   Mon Jul 9 14:03:11 2018 +0200

    Update shadow-cljs and run npm audit fix

src/sass/app.sass

@@ -6,6 +6,17 @@
   main
     margin-bottom: 0
 
+// big loading spinner
+.app-loading
+  display: flex
+  justify-content: center
+  align-items: center
+  height: 100vh
+  font-size: 4.8rem
+  color: $grey-light
+  .loader
+    +loader
+
 // navi on the left side
 .sidebar
   min-height: 100vh
@@ -87,11 +98,11 @@
 @keyframes you-spin-my-head-right-round
   from
     transform: rotate(0deg)
-    transform-origin: 50% 48%
+    transform-origin: 49% 50%
 
   to
     transform: rotate(359deg)
-    transform-origin: 50% 48%
+    transform-origin: 49% 50%
 
 .loading-spinner
   .icon