Add attribution metadata to asset catalogue

[davetaz]

closes #1620

Proposed changes

This set of changes adds three fields to the asset catalogue so that users can properly record the source, attribution and licence for using assets.

These have been added to the data model, interfaces in the authoring tool as well as the import/export plugins.

Checklist

Below is a checklist of the key things that the core team will be looking for when reviewing your PR. Please check off as many of these as you can prior to submitting the PR, but don't worry if you can't do all of them, the core team are here to help!

• [Y] I have read the CONTRIBUTING doc
• [Y] I have checked that the code compiles correctly
• [Y] I have checked that unit test suite passes locally with my changes
• [n/a] I have added new tests that cover my changes (where appropriate)
• [Y] I have added documentation (where appropriate)
• [-] Any dependent changes have already been merged