Author: Ankit Kumar Agarwal
MDL-28483 URL adding validation checks and changing field externalurl to not-null
http://git.moodle.org/gw?p=moodle.git;a=commit;h=6bcfb5f23b06f2859c6f69966d071e4495047d40
http://github.com/moodle/moodle/commit/6bcfb5f23b06f2859c6f69966d071e4495047d40
+ 2.1 en [invalidurl,mod_url]