More robust creation of new gallery filters

[brian-rose]

Currently the dropdown lists on filterable tags are hard coded in the javascript code. We had "domains" and "packages". We are adding a new "events" category in #222.

Ideally these categories would not need to be hard coded, but instead responsive to whatever is in the html. We can add new categories dynamically at gallery build time just by including a new heading in at least one cookbook repo's _gallery_info.yml file. But the filtering will not work until the heading name is hard coded in the javascript.

This is something we should try to fix as part of our overhaul of our website.