Allow sorting by blocking issues and popularity via GraphQL API

What does this MR do?

Resolves #327243 (closed) and #327245 (closed).

For #327243 (closed)

• Allow sorting issues by blocking issues count in asc/desc order.
• Expose blocking_issues_count as blockingCount field on Issue type.

For #327245 (closed)

• Allow sorting issues by popularity (upvotes) in asc/desc order.

Sample query

Sorting by blocking issues:

query {
  project(fullPath: "gitlab-org/gitlab") {
    issues(sort: BLOCKING_ISSUES_DESC) {
      nodes {
        id
        title
        blockingCount # newly added field
        blocked
        blockedByCount
      }
    }
  }
}

Sorting by popularity (upvoted):

query {
  project(fullPath: "gitlab-org/gitlab") {
    issues(sort: POPULARITY_DESC) {
      nodes {
        id
        title
        upvotes
      }
    }
  }
}

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.