Ember.js: Add assets img + css (ember-hacks.css) to ember admin

Closes #2276

- Adds the images to `core/client/assets/img`
- Adds css with ember hacks to `core/client/assets/css`
- Configures middleware to provide assets as static files at url `/ghost/ember`
- Adds ember option to assets helper
- Modifies default-ember.hbs to use ember option on asset helper
- Remove inline style definition in editor.hbs
- Modifies .gitignore to include ember-hacks.css (in ignored `core/client/assets/css` folder)

ghost/admin/assets/css/ember-hacks.css

@@ -0,0 +1,20 @@
+/*
+  Cosmetic changes to ghost styles, that help during development.
+  The contents should be solved properly or moved into ghost-ui package.
+*/
+
+.post-settings-menu {
+    display: none !important;
+}
+
+#entry-markdown,
+.entry-preview,
+.CodeMirror.cm-s-default {
+    height: 500px !important;
+}
+
+.editor input {
+    -webkit-transition: none;
+    -moz-transition: none;
+    transition: none;
+}

ghost/admin/templates/editor.hbs

@@ -1,43 +1,3 @@
-<style>
-/* Put your CSS here */
-/*
-@keyframes domChanged { from { background: yellow; } }
-@-webkit-keyframes domChanged { from { background: yellow; } }
-.ember-view { animation: domChanged 1s; -webkit-animation: domChanged 1s; }
-*/
-
-@font-face {
-    font-family: "Icons";
-    src: url("https://testblog111.ghost.io/ghost/fonts/icons.woff") format('woff');
-}
-/*
-  Cosmetic changes to ghost styles, that help during development.
-  The contents should be solved properly or moved into the other assets.
-*/
-.post-settings-menu {
-    display: none !important;
-}
-
-#entry-markdown, .entry-preview,
-.CodeMirror.cm-s-default {
-    height: 500px !important;
-}
-
-.editor .entry-title {
-    box-shadow: none !important;
-    background: none !important;
-    padding: 0 !important;
-    height: auto !important;
-}
-
-.editor input {
-    -webkit-transition: none;
-    -moz-transition: none;
-    transition: none;
-}
-</style>
-
-
 <section class="entry-container">
     <header>
         <section class="box entry-title">