Use ClustersController as base while having Projects::ClustersController
to inform what `clusterable` is. Thanks @ayufan for the great suggestion
!

- View changes to work with new approach

- Fix javascript for new approach

- Fix feature specs for new approach

- Fix QA

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/

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

Part of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4110