Add a filter whether registration is enabled or not to find "drop-in" events

Problem/Motivation

A client would like to let visitors search for "drop-in" events distinctly from those that require registration. Currently, the Registration Availability filter shows any events with registration enabled and spots available but there is no filter that allows searching for events that don't require any registration.

Proposed resolution

Add a simple boolean filter for Views that allows filtering by whether Enable Registration is toggled off or on for an event series.

Remaining tasks

Build the filter.