Display last update date on assets page (and maybe sort by it)

[TimJentzsch]

Displaying the Bevy version of an asset is already a great tool to determine if an asset is still up-to-date. Unfortunately, it doesn't work for all projects (see #475).

We could display the date of the last crate update (or last commit) to get another indicator of the maintenance status of a crate. This should be easier to apply to all assets equally (except blogs and tutorials). We could then even sort the assets by these dates, so that outdated assets are naturally pushed back in the list.