Migration archive: In issue events, write out label as URL, milestone as "milestone_title", "milestone_number"

The "labeled" event can also include attributes like this:
```
issue_events_000006.json-    "label_name": "help wanted",
issue_events_000006.json-    "label_color": "008672",
issue_events_000006.json-    "label_text_color": "ffffff",
```