Allow events to have a non-physical address

**User story:**
Not all events are in person; our authors need a way to create an event that is a webinar. Currently a because a physical address is required for events, so this type of event has to be created in a different content type.

We might want to consider making the address non-required; or, we might want to add another address type choice and allow authors to enter a URL for that type.

**JIRA ticket #:** DP-8109

No implementation files, tests, or entry points are identified in the request. First clarify whether events should allow no address or support a URL address type, then identify the event content type and its authoring and validation paths; done means webinar events can be created without requiring a physical address.

Written by the indexing model from the issue text.