Try to simplify feature flag checks by using policies

There are two cluster hierarchies one for the deployment platform and
one for controllers. The main difference is that deployment platforms do
not check user permissions and only return the first match.

Instance level clusters were already mostly supported, this change adds
admin area controllers for cluster CRUD

It is important to destroy data related to repo mirrors when they are
disabled.
Use `_destroy` nested attribute instead of `enabled` for push mirrors.
Call `remove_import_data` after saving a project if its pull mirror is
disabled.

Add env_var and file as supported types for CI variables. Variables of
type file expose to users existing gitlab-runner behaviour - save
variable value into a temp file and set the path to this file in an ENV
var named after the variable key.

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/46806.

Session stored globally per request

See merge request gitlab-org/gitlab-ce!27658

When this option is enabled, GitLab will create namespaces and service
accounts as usual. When disabled, GitLab wont create any project
specific kubernetes resources

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56557

- Adds an endpoint on PipelinesController
- Adds a service that iterates over every build in a stage and
plays it.
- Includes 'play_manual' details on EntitySerializer
- Builds a new Stage state: PlayManual. An stage can take this status if
it has manual builds or an skipped, scheduled or manual status
- Includes FE modifications and specs

This is now entirely handled by `create_note`:

1. Project snippets prevent `create_note`.
2. Uploads already only support routing for personal snippets.

This simplifies some policies and access checks, too!