Prefill POST webhook address via a URL query string (BB-8222)

It would be very nice if we could pre-fill the POST webhook URL by passing our URL in the query string:

https://bitbucket.org/Stavros/django-annoying/admin/services?post_url=http%3A//local.getinstabot.com/webhooks/bitbucket/%3Ftoken%3DFGL5RLtKf6pprUCLfuc697

And this would add a POST webhook with the URL field prefilled. This way, we could just link our users to the page, rather than tell them to copy-paste the URL.