Implement asset version history

[Calinou]

This can be used by asset uploaders to give more information to users.

Multiple versions could be stored per asset, each with a version identifier, a date and some release notes (with possible Markdown formatting).

We could make old versions downloadable manually via the Web interface, but the editor integration would likely only provide the latest version for simplicity's sake.

[Calinou]

This is now implemented on the backend, but old versions remain to be displayed on the Web frontend.

I'm not sure about release notes though – maybe letting people link to a CHANGELOG.md file is sufficient. In general, I'm in favor of moving data to the repository so it can be easily viewed by anyone (including those finding the asset directly on GitHub or similar).

[Calinou]

The version history is now displayed on the Web frontend and API, closing.