[#843] Add "status: claimed" when someone claims an issue

There's no push notification for when someone does it using the
dropdown, but this will work for most people because most people can't
select from a dropdown anyway
Add response text for easier debugging

(can be viewed in Github -> Settings -> Hooks -> your webhook -> Recent
Deliveries -> Response)
[#843] Add status: unclaimed / status: claimed automatically

* only to type: issue, not type: pull request

* when the issue is opened, apply the claimed/unclaimed status

* when a comment is made on an issue, update the claimed/unclaimed status
  if it's been changed without our noticing (may have been selected via
  the dropdown, for example), and make sure to automatically add the
  claimed/unclaimed status if the labels have been edited
[#843] Assign / add status: claimed when someone makes a pull request

Includes some refactoring of common functionality into a function
Merge pull request #72 from afuna/github-claim-labels-bug-843

Automate status: claimed/status: unclaimed