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

In preparation so that we can create both cluster attached to project
and cluster attached to group.

- Move ClustersController to top level

- Move Clusters::ApplicationsController to top-level too

- Creates a Clusters::BaseController to share common functions

- Do not rely on @project ivar. Anything could set the ivar.

- Fix Vue page components due to new data-page value

Because of the controller change we have gone from
`projects:clusters:new` to `clusters:new`, so we need to update the file
location of the page components. There is somewhere a function that will
convert data-page to a file location.

On that note, projects/clusters/gcp/new/, translate to
Projects::Clusters::Gcp#new doesn't exist so replace that with
clusters/create_gcp/ and clusters/create_user/

The RBAC section header was changed, causing the anchor to change as
well. For our first release with RBAC support, it is important
we get the link right.

Now that Auto DevOps can run in a RBAC enabled cluster from
https://gitlab.com/gitlab-org/gitlab-ce/issues/51942, we can now remove
the FF.

As the flag only governed if UI elements would appear or not when
creating/adding existing clusters; the effect of removing the FF would
be that the checkbox to choose to enable RBAC clusters will now always
appear.

Remove FF stubs from specs

Improve spec context names

This is similar to the option in the Add Existing cluster form

# Conflicts:
#	app/controllers/projects/clusters/gcp_controller.rb
#	app/views/projects/clusters/gcp/_form.html.haml

Add GKE, Login - HTML & CSS
Cluster details - HTML & CSS

Fixed HTML