GraphQL: Create an issue

Add a mutation to create an issue via GraphQL for the purposes of Issue Boards. This means that we need to make sure that we have the ability to set the list attributes: label, milestone and assignee.

REST equivalent:

POST /group/project_path/-/issues

issue[title]: great issue
issue[description]: it's awesome
issue[confidential]: 0
issue[assignee_ids][]: 1
issue[epic_id]: 11389
issue[milestone_id]: 61
issue[label_ids][]: 
issue[label_ids][]: 115
issue[label_ids][]: 116
issue[weight]: 1
issue[due_date]: 
issue[lock_version]: 0