HostingService subclasses can now provide a set of instructions for

configuring any incoming webhooks that their repositories support. This

is done by setting has_repository_hook_instructions = True on the

subclass and then overriding get_repository_hook_instructions().
Any repositories whose HostingServices support hook instructions will

have a "[Hooks]" link in the repository list page in the admin UI.

Clicking this will pop up a modal dialog with the instructions.
Currently, this is only implemented for GitHub. The GitHub instructions

explain the close-submitted webhook, provide examples, link to the page

for adding a new webhook for that repository, and provide all the fields

to fill in.
The other supported services will come soon.

Tested with several different repositories. Verified that the links worked

and all the sample values matched what I'd expect.