Add project_name, project_version, url to Slack notification context

[leplatrem]

This would allow to define the notification as ✨ collection {collection_id} was created on {settings[project_name]} instead of having to duplicate the notification definition on each environment.

See also Kinto/kinto-emailer#510

[leplatrem]

Oh, no, we have '[{settings[project_name]}] in kinto-emailer
update: replaced {project_name} with {settings[project_name]}