Namespace github webhooks

[fsaintjacques]

The goal is to selectively toggle prod instance without touching github's repository setting. (They'll just 4xx if we disable it).

# http://nashville.ursalabs.org:4100/change_hooks/github_ursabot
if conf.github.event_hook.enabled:
    c['www']['change_hook_dialects'] = {
        'github_ursabot': {
            'class': GithubHook,
            'token': conf.github.token,
            'secret': conf.github.event_hook.secret,
            'debug': conf.github.event_hook.debug,
        }
    }
# http://nashville.ursalabs.org:4100/change_hooks/github_arrow
if conf.github.event_hook.enabled:
    c['www']['change_hook_dialects'] = {
        'github_arrow': {
            'class': GithubHook,
            'token': conf.github.token,
            'secret': conf.github.event_hook.secret,
            'debug': conf.github.event_hook.debug,
        }
    }

[kszucs]

Unresolvable, see comment in #61