proof of concept improving user info experience

Currently, user info display for actions results, like `Content updated` are displayed by a `alert` JS function.

As you already use bootstrap, my proposition is to replace these old-time alerts by the bootstrap ones.

See screencast below, if you're happy with it I will change it for all the alerts.

![img](http://g.recordit.co/k9tiFR3PLI.gif)
I love that this is in `layout.mustache` and therefore available everywhere. Nicely done.