Extension repository should display release dates

but also for the actual main live data of available extensions.

Note that we shouldn't add too many columns since the table is already very large. And we've discussed adding the "supported by" information too (currently not listed). So if we want to add that column, I'd re-examine what's more important between "last release date" and "last modified". And for consistency, I'd replace "Creation Date" with "First release date".