Automatically add merged component-update tickets to the latest milestone

[dscho]

To help with Git for Windows' maintenance, let's relieve human operators from the mental burden to remember at all times to add the appropriate milestone when closing a component-update ticket by merging the respective PR.

[rimrul]

Looking into this a little, it looks like we need the closed action of the pull_request webhook and get the issue from the issue_url, because the issues webhook has seemingly no easy way to tell wether an issue was closed by hand or by merging a PR.