T12124

* Grouping resources list by categories
* Renaming Category to TagCategory
* Creating ResourceCategory model + db table + CRUD
* Removing ResourcesCategory enumeration
* Creating DB migration to fix old resources categories
* Updating db/seeds + updating organisations.csv
* Including 'public/system' directory to .gitignore file
* User should not be able to remove the tag_category
* Improving specs + cleaning code