Remove get_taxonomy_registration_args method from event and venue cla…

[mauteri]

Description of the Change

This is a refactor of code and some changing of responsibilities.

• Removed get_taxonomy_registration_args methods and handled post type/taxonomy/post meta registrations in related classes, not setup class.
• Created Topic class to manage that taxonomy outside the Event class.
• Created Event_Setup class to handle registration and event functionality that affects event setups outside of individual events. This removes some odd static methods from the Event class and lets it behave as an instance of a single event.
• Moved some static methods out of Event class and into Utility class where they make more sense.
• Added and update unit test.

Closes #564 #563

How to test the Change

Code should function same as it had before.

Changelog Entry

Changed - Existing functionality

Credits

Props @mauteri

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.