Update existing attachments

* [x] Major clean-up of JavaScript
* [x] Major clean-up of templates
* [x] Major clean-up of views
* [x] Removed the two-steps form submission (pre-validation is now done using HTML5)
* [x] Clean-up the form next mechanism
* [x] Changed french translation for attachment "fichier lié" --> "fichier attaché"
* [x] Add ability to edit existing attachments