Get all Events

[etkndr]

• [x] Seed data exists in the database for events to be returned.
• [x] Successful response includes each event in the database.
• [x] Event data returned includes the id, groupId, venueId, name, type, startDate, endDate, and previewImage
• [x] Event data returned includes aggregate data for numAttending
• [x] Event data returned includes associated Group data, including id, name, city, and state
• [x] Event data returned includes associated Venue data, if any, including id, city, and state