File system overhaul

# Todo
- [x] Replace folders with categories
- [x] Ensure that files can only be loaded when user has permissions to do so
- [ ] Re-do acp file managment
   - [x] Quick navigation between different categories
   - [ ] Specify category when uploading a file instead of forcing currently open category as file category
   - [ ] Fetch file details on demand
- [ ] Re-do file picker dialog
- [ ] Remove `\cms\data\file\File::getByID()`

# Planning
- [ ] Should it be possible to upload a file without a category? We could create a default category during the installation and drop the overhead of handling files without a category.
- [ ] Should it be allowed to assign a file to more then one category?
- [ ] Should the file selection of the file and image content type be replaced with selecting a category instead? Allows an easier editing when a set of files is used across multiple pages but it might be confusing where to edit the actual set of files since the content type forms would only name a category.