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 short commit message may be truncated making the truncated
portion unreadable. The title of the link should be the commit message
so that the commit message is shown when hovering the commit message
anchor.

Closes gitlab-org/gitlab-ce#42673

- Allow to extend nav links for the Group sidebar

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

Replaces .card-header > hX.card-title with hX.card-header to remove
.card-title. At a occurrence where bold styling is not necessary,
div.card-header is used instead.
By full removal of .card-title at the moment, all workarounds for
BS4's card-title are removed as well.

More information: https://getbootstrap.com/docs/4.0/migration/#panelsSigned-off-by: NTakuya Noguchi <takninnovationresearch@gmail.com>

This removes the `ForkedProjectLink` model that has been replaced by
the `ForkNetworkMember` and `ForkNetwork` combination. All existing
relations have been adjusted to use these new models.

The `forked_project_link` table has been dropped.

The "Forks" count on the admin dashboard has been updated to count all
`ForkNetworkMember` rows and deduct the number of `ForkNetwork`
rows. This is because now the "root-project" of a fork network also
has a `ForkNetworkMember` row. This count could become inaccurate when
the root of a fork network is deleted.

When the feature is available, this setting allows admins to choose a
project as a source of custom file templates. This is in addition to
any instance-wide templates, whether custom or vendored into the GitLab
codebase.

The MR below adds a test for the code owners feature. This adds the
part of those changes specific to CE - the ability to add and view
files in a project.

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7368

Currently, we assume a "last_version" always exists for a wiki page.
In production, this is not always true. So, guard uses of it with a
null check.

Before this commit updating Prometheus (e.g. adding alerts) made it
"updated" therefore not installed.

When a project is disabled, visiting the home page would show,
"An error occurred while loading commit signatures". This change checks
that the user has permission to view the project to avoid unnecessary
GPG signature lookups.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50903

Signed-off-by: NRémy Coutable <remy@rymai.me>