milestoned and demilestoned events handling

So, github sends the "milestoned" and "demilestoned" events (setting or unsetting the milestone) for pull requests as issue events.. thus the rules for peril don't quite work as all will show up as issue events even if they are pull requests.

I believe we can perform a "workaround" though.
in the payload under "issue" there is a pull_request node that is set if the issue is an actual PR. So we could use that to detect that it is, in-fact, a PR and fake the event to be pull_request instead.

I'm thinking (from skimming the code) that this might be doable in github_runner.ts#runsForEvent and "correct" the event as pull_request

abbreviated webhook payload for milestoned
```javascript
{
"action": "milestoned",
"issue": {
"number": 123,
"title": "my PR title",
// other attributes (assignees, labels, user, etc..updated_at, created_at, etc..)
"milestone": {
// new milestone of PR
},
// how we can detect it is a pull request
"pull_request": {
// various pull request URLs
},
"body": "my description"
},
"repository": {
// repository details
},
"organization": {
"login": "myorg",
// other org data
},
"sender": {
"login": "sender",
// other sender data
},
"installation": {
"id": 12345
}
}
```