Add a "maintenance status" property to assets

[Calinou]

Inspired by https://doc.rust-lang.org/cargo/reference/manifest.html#the-badges-section. We can probably reuse the categories as-is, except for Experimental which would be replaced by the Testing support level we already have.

While this is purely declarative, this could help people make better decisions when choosing an asset.

This maintenance statement would be displayed on the web frontend as a notice of the form "According to its author, this asset is actively developed."

[Ansraer]

Is there any reason why we couldn't change the state of an asset automatically depending on when it (or a linked git project) was last updated?

[Calinou]

Is there any reason why we couldn't change the state of an asset automatically depending on when it (or a linked git project) was last updated?

We can display a notice for assets that haven't been updated for more than a year, but a maintenance status is more than just a "last update" thing. It's something filled on a voluntary basis to help people make better decisions. Some smaller assets don't really need to have more than one release per year.