Add epic filter and sort by last created or updated

What does this MR do and why?

Adds the ability to sort a collection of epics by created_at or updated_at, ascending or descending.

How to set up and validate locally

1. create some epics in a group
2. In GraphiQL, use the query

query {
	group(fullPath: "gitlab-org") 
  {
    epics(createdBefore: "2020-01-21", createdAfter: "2020-01-10", sort: UPDATED_AT_ASC, first: 5) {
      nodes {
        title
        createdAt
        updatedAt
      }
    }
  }

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #351981 (closed)