Allow attendee to appear anonymous in attendee lists for events

[mauteri]

Description of the Change

This change adds a setting to global event settings to enable anonymous RSVPs for events. In addition, each event can toggle whether the event will allow anonymous RSVPs.

When enabled, a member RSVPing to an event has the choice to show up listed as anonymous. Only users with edits_posts capability can see the identity of the member, all other members and non-members will see the person as anonymous.

If the person chooses to change attendance to "not attending" and they are anonymous, the record will be removed and not placed in "not attending" view. There's a few reasons for this.

• Someone anonymous that is not attending holds no value.
• In the rare case the anonymous RSVP feature is disabled for the event that members have already responded to, we do not want to reveal the member's identity.

In the situations that anonymous RSVP is disabled after a member marks them self anonymous, the anonymous feature will remain for that person.

Closes #103

How to test the Change

• Deactivate/Reactivate plugin for database updates.
• Global setting is enabled by default, so new events will have anonymous RSVP enabled.
• If Global setting is disabled, new events will not have anonymous RSVP checked by default.
• If enabled, checkbox to be anonymous will be included in RSVP modal.

Changelog Entry

N/A

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.