Author: Dongsheng Cai
MDL-28666 Extends repository to support references
1. Allow repository to create references to external contents
2. Extends files API to handle file references
3. Generic file caching
4. Backup/restore file references
5. Download external contents if repository uninstalled
6. Allow filepicker to display iframe
7. PHPUnit test suits
http://git.moodle.org/gw?p=moodle.git;a=commit;h=6723372514e0bf8b44b945b4a625ac43e08dc382
http://github.com/moodle/moodle/commit/6723372514e0bf8b44b945b4a625ac43e08dc382
+ 2.3 en [filereferencesincluded,core_backup]
+ 2.3 en [filereferencessamesite,core_backup]
+ 2.3 en [filereferencesnotsamesite,core_backup]
+ 2.3 en [includefilereferences,core_backup]
+ 2.3 en [externalfilenolocation,core_error]
+ 2.3 en [confirmdelete,core_repository]
+ 2.3 en [confirmremove,core_repository]
+ 2.3 en [continueuninstall,core_repository]
+ 2.3 en [continueuninstallanddownload,core_repository]
+ 2.3 en [invalidparams,core_repository]
+ 2.3 en [uselatestfile,core_repository]