Limit the width of images in emails

This fixes an issue in Mail.app, where large images would expand outside
of the viewport.

app/views/layouts/notify.html.haml

@@ -4,6 +4,10 @@
       %title
         GitLab
   :css
+    img {
+      max-width: 100%;
+      height: auto;
+    }
     p.details {
       font-style:italic;
       color:#777