Add issue type GraphQL query for Issues

As mentioned in #229544 (comment 383559891), we need to be aable to query issues by issue_type.

Currently there are two types:

• issue
• incident

Excluding the issue type filter should show all issues.

Existing query:

query getIncidents($projectPath: ID!, $labelNames: [String], $state: IssuableState) {
  project(fullPath: $projectPath) {
    issues(state: $state, labelName: $labelNames) {
      nodes {
        iid
        title
        createdAt
        labels {
          nodes {
            title
            color
          }
        }
        assignees {
          nodes {
            username
          }
        }
      }
    }
  }
}