File system overhaul

# Todo
- [x] Replace folders with categories
   - [x] Assign files to more then one category
   - [x] Files no longer can have no category. Instead, we create a default category during the installation of Fireball CMS.
- [x] Ensure that files can only be loaded when user has permissions to do so
- [x] Re-do acp file managment
   - [x] Quick navigation between different categories
   - [x] Specify category when uploading a file instead of forcing currently open category as file category
   - [x] Fetch file details on demand
   - [x] Clipboard Actions
   - [x] Sorting
- [x] Re-do file picker dialog

# Planning
- [x] 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. **The current system with selecting the individual file remains.**