As an organizer, I want to be able to add the topic of my event, so that my event can be found by interested potential participants via topic search on an Event List or via a future Event Notifications feature.

Background: We currently do not store any data related to event topic. However, there is an interest in topical calendars (for example, all gender-related events, all sustainability-related events) in the movement. As a first step, we will need to determine what some of these topics may even be. We will need to coordinate with many teams, including Grants, Community Growth, Research, and others to make this determination. While this ticket is not yet ready for estimation, it is a placeholder for us to document what we learn.