Notification of string changes

Author: Marina Glancy
MDL-61477 admin: sitepolicy handler API
- Define sitepolicy handler manager class, base class and the core handler
- Allow to set a plugin as sitepolicyhandler that implements the sitepolicy API
- Modify web services to return information from the 3rd party handler instead of core if needed


http://git.moodle.org/gw?p=moodle.git;a=commit;h=aef659d1e2f39545017fa3bd54a158fdfb676bb5
http://github.com/moodle/moodle/commit/aef659d1e2f39545017fa3bd54a158fdfb676bb5

+ 3.4 en [policysettings,core_admin]
+ 3.4 en [privacyandpolicies,core_admin]
+ 3.4 en [privacysettings,core_admin]