BE: Expose blocked and blocking work items for EpicType

Summary

As we are updating relationship icons across epic and issue boards and lists, we need to expose all WIT (issue, task, objective, key result) that can be blocked by or blocking an epic. Currently on epic boards we are unable to query any blocking/blocked items that are not epic types. frontend only gets epic type blockers/blocking items on epic board via app/assets/javascripts/vue_shared/components/issuable_blocked_icon/graphql/blocking_epics.query.graphql:

This issue also closely relates to #499362 (closed).

Proposal

Step 1. Add a new field to EpicType called blocked_by_work_items that returns a list of work items that are blocking the epic

Example query:

query epicBlockingItems {
  group(fullPath: "flightjs") {
    name
    epics(iid: "18") {
      edges {
        node {
          name
          blockedByWorkItems {
            edges {
              node {
                name
              }
            }
          }
        }
      }
    }
  }
}

Step 2. Add a new field to EpicType called work_items_blocked_by_epic that returns a list of work items that are blocked by the epic

Example query:

query itemsBlockedByEpic {
  group(fullPath: "flightjs") {
    name
    epics(iid: "16") {
      edges {
        node {
          name
          workItemsBlockedByEpic {
            edges {
              node {
                name
              }
            }
          }
        }
      }
    }
  }
}