LPS-49798 add column to store the original URL of an article

For articles whose authoritative origin isn't KB itself, store the
source location in this new column. The value of this source location is
expected to be any valid URL.

This can be used to identify and offer extended functionality for
articles imported from Github, Bitbucket, etc. For the moment, only
support for GitHub will be implemented.
LPS-49798 expose new field in services
LPS-49798 add field to edit form
LPS-49798 disable source location by default
LPS-49798 add ribbon when article has source URL
LPS-49798 make lang key configurable
LPS-49798 use Validator.isUrl method for validation
LPS-49798 autogenerated