Show the image button only if editing admin pages

That's the only place where we need to attach images so far.

app/assets/javascripts/ckeditor/config.js

@@ -94,8 +94,12 @@ CKEDITOR.editorConfig = function( config )
     { name: 'paragraph', groups: [ 'list' ], items: [ 'NumberedList', 'BulletedList' ] },
     { name: 'links', items: [ 'Link', 'Unlink' ] },
     { name: 'styles', items: [ 'Format' ] },
-    { name: 'insert', items: [ 'Image' ] },
     { name: 'basicstyles', groups: [ 'basicstyles', 'cleanup' ], items: [ 'Bold', 'Italic', 'Underline', 'Strike' ] }
   ];
+
+  config.toolbar_admin = config.toolbar_mini.concat([
+    { name: 'insert', items: [ 'Image' ] }
+  ]);
+
   config.toolbar = "mini";
 };

app/views/admin/site_customization/pages/_form.html.erb

@@ -50,7 +50,8 @@
 
       <div class="ckeditor margin-top">
         <%= f.label :content %>
-        <%= f.cktext_area :content, label: false, cols: 80, rows: 10, ckeditor: { language: I18n.locale } %>
+        <%= f.cktext_area :content, label: false, cols: 80, rows: 10,
+                          ckeditor: { language: I18n.locale, toolbar: "admin" } %>
       </div>
 
       <div class="small-12 medium-6 large-3">