Issue for tracking the progress of ckanext-files integration into CKAN

• Phase 1: add file keeper and use it in uploaders instead of direct interaction with FS
• Phase 2: expose storage configuration for native uploaders. Add basic support for cloud uploads
• Phase 3: add models for tracking files and API actions. Native uploaders are not using them yet, to be compatible with custom uploaders
• CKAN v2.12: At this point, custom uploaders are still supported, but now they can be rewritten to use new storages
• Phase 4: drop IUploader interface
• Phase 5: start tracking files uploaded into resources, groups, and user profiles.