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)

Author: Manuel Mitasch

.gitignore

@@ -38,6 +38,7 @@ projectFilesBackup
 /core/clientold/assets/fonts
 /core/clientold/assets/vendor
 /core/client/assets/css
+!/core/client/assets/css/ember-hacks.css
 /core/client/assets/fonts
 /core/server/data/export/exported*
 /docs

core/client/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;
+}