To the extent possible swap out `project` with `clusterable`
- Abstract paths for showing cluster or clusters. This will allow us to
swap in alternative paths for group level cluster
- Push :project_id and :namespace_id params from the URL to the POST
body.
- Create a nice helper for to generate links for the destroy
action
For some reason, spec :project_id and :namespace_id param are not going
through `to_param` for a JSON format. Manually call `to_param` to fix
specs.
- Move :layout to BaseController
