Refactor product form (association with categories)

Todo
- [x] fix issue with category removal
- [x] refactor javascripts to keep only one associate script (one for single and one for mass action for now)
- [x] refactor form type to use the same field for enrich and import

Bug fix: [no]
Feature addition: [no]
Backwards compatibility break: [yes]
Unit test passes: [yes]
Behat scenarios passes: [yes]
Checkstyle issues: [no]