Uses a new factory method to DRY instantiation.

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8731

This reverts commit 427c1e72, reversing
changes made to 5aac64a7.

Show a dismissable callout on a users dashboard for a free trial of
the GitLab.com Gold plan. Hide the callout from users who have
dismissed the callout, are already on a trial or are already on the
gold plan

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 reverts merge request !21551

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