Ember flash message

**DO NOT MERGE YET**

This PR replaces the custom flash implementation with a modified version of [ember-flash-message](https://github.com/ryanto/ember-flash-message/).

It needs PR https://github.com/ryanto/ember-flash-message/pull/19 to be merged.

I also fixed some case where flash were not displayed (with both implementation).

As soon as the PR is merged, I'll replace the first commit (with the `FIXME` notice) with a proper bower dependency setup.