WORK IN PROGRESS: feedback 

The goal of this PR is to have a discusion for the architecture of the feedback component.
Currently, the html part is made in index.html and a controller gaFeedbackController is supposed to manage the UI and the submission. 
@elemoine Does it make sense ? Should I create a directive ? Currently, we have no "feedback" component. I could imagine to have a component with a form directive.
Thanks in advance for your feedback.