Merge pull request #15475 from mecachisenros/externUrl

dev/core#1100 Add externUrl method and civicrm_alterExternUrl hook

diff --cc CRM/Contribute/Form/ContributionPage/Widget.php
Simple merge

diff --cc CRM/Cxn/BAO/Cxn.php
Simple merge

diff --cc CRM/Mailing/BAO/Mailing.php
Simple merge

diff --cc CRM/Mailing/BAO/TrackableURL.php
Simple merge

diff --cc CRM/Utils/System.php
Simple merge