Add some QoL improvements to `EventLog` admin

[joshuadavidthomas]

• Search by:
  • Event name (e.g., "pull_request", "issues")
  • Repository or owner name (from payload)
  • Payload contents
• Add list filters for:
  • Event type
  • Action (from payload)
  • received_at date filtering ("Today", "This Week", "This Month")
• Add more fields to list table:
  • Installation ID
  • Repository name
  • Sender/actor username
• Default ordering by -received_at (make explicit)