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/

Remove the inheritance from ::BaseService which is causing us to inherit
the initializer that has project as the first arg, as we will not have
access to project with forthcoming group clusters.

Also call install service from create service - 1 less thing to re-use

Extract TestRequest code into a spec helper. Given that we need
different behaviour for Rails 5.0 (and again in Rails 5.1!), it's handy
to have that branching behaviour in one place

This should help with code re-use when we create applications for group
level cluster next.

Change `find_or_initialize_by` to explicitly find or build the right
association based on the application name. The benefit here is that we
use the associations on @cluster rather than querying from the other
side of the association.

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 whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>