Allow sorting issues by popularity in GraphQL API

Summary

The issue list can be sorted by popularity. We should add the ability to do this to the GraphQL API.

There are some sort options already available, e.g. UPDATED_DESC.

This update should enable the following queries:

query {
  project(fullPath: "gitlab-org/gitlab") {
    issues (sort: POPULARITY_ASC) {
      count
    }
  }
}

query {
  project(fullPath: "gitlab-org/gitlab") {
    issues (sort: POPULARITY_DESC) {
      count
    }
  }
}