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/

This enables us to have other options in the future such as passing in
`group:`.

Also, remove project arg from UpdateService, as un-used.

This will help group cluster controller to re-use this services.

Enables frozen string for some vestigial files as
well as the following:

* app/controllers/projects/**/*.rb
* app/controllers/sherlock/**/*.rb
* app/controllers/snippets/**/*.rb
* app/controllers/users/**/*.rb

Partially addresses #47424.

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
#	app/views/projects/clusters/gcp/login.html.haml