Use comments table for RSVP instead of custom RSVP table.

[mauteri]

Description of the Change

This change utilizes the Comments table with a comment type of gatherpress_rsvp in WordPress instead of a custom RSVP table. It also adds code for a comment tax_query, which isn't used at the moment, but will likely come in handy soon. This requires an update to GatherPress Alpha to migrated old RSVPs in custom table to Comments table, then delete the custom table.

Closes #687

How to test the Change

• RSVP should work as it always has, but RSVPs will now be stored in Comments table.
• A custom taxonomy _gatherpress_rsvp_status is also being used for attending, not_attending, waiting_list
• Old RSVPs should not appear and need to be migrated with a change in GatherPress Alpha plugin.

Changelog Entry

Changed - Existing functionality

Credits

Props @mauteri, @carstingaxion

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.

[what-the-diff]

PR Summary

• Enhancement in User Email Utilization
  Temporarily incorporated a feature to gather and use user email information across various components of our platform.

• Improved Comment Management
  Included a new code that fetches comments and either inserts them as new or updates the existing ones based on their presence.

[stephenerdelyi]

Just tested this by:

• reviewing that my existing test events (with attendees previously) are no longer displaying upon checkout + build
• signing up for a test event from the frontend
• ensuring that user was recorded
• added a test user under a different account
• successfully deleted / managed users from the gutenberg backend

No unexpected results were produced as a result of these updates. Marking as approved, though we should plan to update alpha to migrate over the existing data 🎉