Add customized instructions for configuring webhooks for repositories.

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.

Reviewed at https://reviews.reviewboard.org/r/6314/