Epics API: Title, description, and dates

• For epic objects, implement GET/POST/PUT/DELETE.
• Implementing GET-ing a list of epic objects, similar to https://docs.gitlab.com/ee/api/issues.html#list-issues.
• Same permissions as web UI.
• Only attributes in scope are:
  • Title (required)
  • Description (optional)
  • Planned start date (optional, must be before end date)
  • Planned end date (optional, must be after start date)
• Associated issues are not in scope