Template rendering small fixes and enhancements

This pull request proposes some fixes and enhancements for template rendering, namely:

* In template implementations, show a link to the template it's currently based on (for easy access to it, mainly when we want to configure the template). Maybe a more ellegant solution would be to change the drop-down select, by a button called "Change template".

* When there are no template implementations, display an appropriate message in the template configuration page.

* When we save an implementation without selecting a template in the drop-down, ez-templates will throw an IllegalStateException, which is regarded by Jenkins as a bug. This situation should be handled more gracefully (e.g. display a form validation error). I propose to throw the exception just when we detect a bug in ez-templates.