Fix/improve issue tracker app edit

This Pull Request try to improve the readability of the IssueTracker edit part in App edit form.

 * Start using some Decorator on this form to extract some logic out of the view.
 * Add some acceptance test with Javascript interpretation to be sure the code do what we think.

If some people can review it ?

It's the start to extract all issue tracker from Errbit to some plugin. Can help to do it on the view side now.