Display previous versions download links for an extension

On the page of an extension, we should provide some links to be able to see/download previous versions of an extension.
While previous versions can be easily found for extensions which are uploaded as xar attachments to the page, for extensions that are imported from maven, it is impossible to get a previous version link from the UI. E.g. for this extension http://extensions.xwiki.org/xwiki/bin/view/Extension/Admin+Tools+Application , version 3.0.2 is nowhere to be found.

This is useful if, for a reason or another, extension manager UI does not work and extension needs to be installed manually, or for installing older versions of extensions on older versions of xwiki (which don't have the Extension Manager).