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