change bug tracker links to https://github.com/owncloud/core/blob/master/CONTRIBUTING.md
Merge branch 'master' into fixing-bugtracker-link-master

Conflicts:
	core/templates/exception.php
	settings/templates/help.php
Merge pull request #2020 from owncloud/fixing-bugtracker-link-master

change bug tracker links to https://github.com/owncloud/core/blob/master...