Feature flags: More descriptive name for `boolean` title field

We currently use some confusing language in the ChatOps feature messages. There is a field called boolean which I believe maps to the behavior of the feature flag when no actor is set (i.e. whether it is globally enabled or enabled by default). Here is what it currently looks like:

In this MR I am trying to give this field a more descriptive name; I called it Default Enabled, but I'm not sure that it matches its intent best. Open for suggestions!

I also decided to address this not on the feature gate key level, but just cosmetically "on the way out" when we render gates to Slack attachments, since that seemed the safest thing to do. Internally, the field title is still boolean.