This feature adds standard license metadata fields on every artefact and every
artefact edit page. This allows users to specify the license for the content
they create or upload. For content originating elsewhere, the users can also
enter the name of the licensor and original URL.

Previously, adding license metadata to an artefact or page was achievable in an
ad hoc fashion using a block on a page, but had many limitations.

See also bug #1095499.

This commit changes the database schema and the artefact abstract data type.

Change-Id: Icf9143864a252b43fa91294825b24e5a65c0b4d5
Signed-off-by: Jiri Baum <jiri@catalyst-au.net>