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.

- 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

Updates the EnvironmentController#metrics_dashboard endpoint
to support a "dashboard" param, which can be used to specify
the filepath of a dashboard configuration from a project
repository. Dashboard configurations are expected to be
stored in .gitlab/dashboards/.

Updates dashboard post-processing steps to exclude custom
metrics, which should only display on the system dashboard.

Delete leftover code for referenced_merge_requests endpoint

Closes #60476

See merge request gitlab-org/gitlab-ce!27335

When showing the HTML version of the environment metrics we don't need
to fetch their metrics because we don't use them anymore on the HTML
version.

We use additional_metrics.json endpoint now.

Create dashboards endpoint & setup dashboard post-processing

Closes #60383

See merge request gitlab-org/gitlab-ce!27405

This new table will be used to store the external_dashboard_url which
allows users to add a link to their external dashboards (ex Grafana)
to the Metrics dashboard.

Inside a wiki, when we show the sidebar or browse to the `pages`,
all page contents are retrieved from Gitaly and that is a waste
of resources, since no content from that pages are going to be
showed.

This MR introduces the method `ProjectWiki#list_pages`,
which uses new wiki_list_pages RPC call to retrieve
pages without content

Also in the `WikisController` we're using the method to show
pages in the sidebar and also on the `pages` page.