general: custom file manager for backup storage areas

Once storage has been defined and implemented (MDL-22145), we need to offer the user with some simple interface to those file areas.

Some notes about course_backup / section_backup / activity_backup areas:

• with backup or restore caps on that file area must be able to browse them always
• with moodle/backup:downloadfile will be able to download files
• with moodle/restore:uploadfile will be able to upload files
• with moodle/restore:restorexxx will be able to restore file

About the user_backup area:

• user will be able to upload / download, restore without any limit. The UI will decide where that backup file can be restored.