Add an RSS/Atom feed that lists events

[bernhold]

The standard plugin only works with blog posts. We need to construct a variant that will track changes to collections of interest (events, series, etc.) and perhaps other things.

[bernhold]

https://ideas-productivity.org/feed.xml is the custom-generated feed that we need to work on.

There may also be another feed being generated by the jekyll-feed plugin. That plugin works only on blog posts, so, since we're not using the blog structure at all, it won't get us what we want. We should disable/remove it.

[bernhold]

Disabled the jekyll-feed plugin.

[bernhold]

I think we have a reasonable feed now for events. Not certain yet that it will behave as we want it to when updates occur. So let's watch it for a while before we advertise the feed (add the RSS icon to the footer).

[bernhold]

Once we're happy with the feed, we should also address #73