Improve access to archived versions of Lucene and Solr [LUCENE-5631]

When visiting the website to download Lucene or Solr, it is very difficult for people to locate where to download previous versions. The archive link does show up when you click the download link, but the page where it lives is replaced in less than a second by the CGI for picking a download mirror for the current release. There's nothing there for previous versions.

At a minimum, we need a link to the download archive that's right below the main Download link. Something else I think we should do (which might actually be an INFRA issue, as this problem exists for other projects too) would be to have the "closer.cgi" page include a link to the archives.

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Migrated from [LUCENE-5631](https://issues.apache.org/jira/browse/LUCENE-5631) by Shawn Heisey (@elyograg)

Start with the website's main Download link and the closer.cgi mirror-selection page mentioned in the issue. Trace where the current-release mirror page replaces the archive link, then make the previous-version archive easy to reach below the main Download link and consider exposing it from closer.cgi; verify that both paths remain accessible.

Written by the indexing model from the issue text.