Add ability to include interest groups for wgs-list widget

There has been a request to keep openMobility on the eclipse.org/org/workinggroups/explore.php page. This is being removed from the WG API but is still desired to be included on the explore.php page.

We should have a data attribute to allow us to include IGs. These will be fetched from the interest groups API and the results will be combined together.

/cc @cguindon