Add end-to-end tests of core functionality

[benhsm]

To be tested:

• Calendar
  • Registering for events
  • (Admin) Creating events
• Registering for events
• Event management UI
  • Updating events
  • Assigning volunteers to horses
• (Admin) User table
  • Update
  • Delete
• (Admin) Horse admin table
  • Create
  • Update
  • Delete