Handle sub-tasks

[mpercy]

Sub-tasks are not associated with their parent tasks. We need to add the parent link.

[mpercy]

Actually it looks like sub-tasks are a very specific type of link, and links are all specified using one big "links list" for the whole project instead of per-issue. Sub-tasks have the special link name "sub-task-link" and the "externalId" field must be used to specify the issues being linked.

See JSON File Example here: https://confluence.atlassian.com/adminjiraserver071/importing-data-from-json-802592907.html

{
    "users": [
        {
            "name": "alice",
            "fullname": "Alice Foo"
        },
        {
            "name": "bob",
            "fullname": "Bob Bar"
        }
    ],
    "links": [
        {
            "name": "sub-task-link",
            "sourceId": "2",
            "destinationId": "1"
        },
        {
            "name": "Duplicate",
            "sourceId": "3",
            "destinationId": "2"
        }
    ],
    "projects": [
        ...