Add support for querying epics with GraphQL

What does this MR do?

Allows one to query for epics using a Group GraphQL query.

Example

{
  group(fullPath: "h5bp") {
    id
    path
    fullPath
    epic(iid: 1) {
      id
      iid
      title
      path
      userPermissions {
        adminEpic
        createEpic
      }
      children {
        edges {
          node {
            id
            iid
            title
            state
            path
            reference(full: true)
            relationPath
            createdAt
            closedAt
            hasChildren
            hasIssues
            userPermissions {
              adminEpic
              createEpic
            }
            group {
              fullPath
            }
          }
        }
      }
      issues {
        edges {
          node {
            iid
            title
            closedAt
            state
            createdAt
            confidential
            webUrl
            dueDate
            weight
            path
            reference
            relationPath
            assignees {
              edges {
                node {
                  webUrl
                  name
                  username
                  avatarUrl
                }
              }
            }
            milestone {
              title
              startDate
              dueDate
            }
          }
        }
      }
    }
  }
}

This MR requires (and is based on) https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13246. It has been extracted from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10999

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #10795 (closed)

doc/user/admin_area/monitoring/health_check.md

-# Health Check
+---
+type: concepts, howto
+---
 
-> **Notes:**
+# Health Check
 
+> NOTE: **Note:**
+>
 >   - Liveness and readiness probes were [introduced][ce-10416] in GitLab 9.1.
 >   - The `health_check` endpoint was [introduced][ce-3888] in GitLab 8.8 and was
 >     be deprecated in GitLab 9.1.
 >   - [Access token](#access-token-deprecated) has been deprecated in GitLab 9.4
->     in favor of [IP whitelist](#ip-whitelist)
+>     in favor of [IP whitelist](#ip-whitelist).
 
 GitLab provides liveness and readiness probes to indicate service health and
 reachability to required services. These probes report on the status of the
@@ -17,8 +21,7 @@ traffic until the system is ready or restart the container as needed.
 ## IP whitelist
 
 To access monitoring resources, the requesting client IP needs to be included in a whitelist.
-
-[Read how to add IPs to a whitelist for the monitoring endpoints][admin].
+For details, see [how to add IPs to a whitelist for the monitoring endpoints](../../../administration/monitoring/ip_whitelist.md).
 
 ## Using the endpoints
 
@@ -87,9 +90,8 @@ will return a valid successful HTTP status code, and a `success` message.
 
 ## Access token (Deprecated)
 
->**Note:**
-Access token has been deprecated in GitLab 9.4
-in favor of [IP whitelist](#ip-whitelist)
+> NOTE: **Note:**
+> Access token has been deprecated in GitLab 9.4 in favor of [IP whitelist](#ip-whitelist).
 
 An access token needs to be provided while accessing the probe endpoints. The current
 accepted token can be found under the **Admin area ➔ Monitoring ➔ Health check**
@@ -103,10 +105,21 @@ The access token can be passed as a URL parameter:
 https://gitlab.example.com/-/readiness?token=ACCESS_TOKEN
 ```
 
+<!-- ## Troubleshooting
+
+Include any troubleshooting steps that you can foresee. If you know beforehand what issues
+one might have when setting this up, or when something is changed, or on upgrading, it's
+important to describe those, too. Think of things that may go wrong and include them here.
+This is important to minimize requests for support, and to avoid doc comments with
+questions that you know someone might ask.
+
+Each scenario can be a third-level heading, e.g. `### Getting error message X`.
+If you have none to add when creating a doc, leave this section in place
+but commented out to help encourage others to add to it in the future. -->
+
 [ce-10416]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10416
 [ce-3888]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3888
 [pingdom]: https://www.pingdom.com
 [nagios-health]: https://nagios-plugins.org/doc/man/check_http.html
 [newrelic-health]: https://docs.newrelic.com/docs/alerts/alert-policies/downtime-alerts/availability-monitoring
-[kubernetes]: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-probes/
-[admin]: ../../../administration/monitoring/ip_whitelist.md
+[kubernetes]: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-probes/
\ No newline at end of file