Improve search for GitHub issues

We can specifically search GitHub Issues by doing something like:

"3d viewer" site:github.com/fiji/*/issues

The asterisk seems to do what's expected here. We could provide radio buttons for the various SciJava organizations to limit the results to just issues of that org.

Start by reviewing the search portal's existing search controls and how GitHub issue queries are assembled. Add radio buttons for the relevant SciJava organizations so a selected organization limits results to that organization's GitHub issues, and verify that the existing search still works without a restriction.

Written by the indexing model from the issue text.