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.

Group project templates should be a Silver/Premium feature - CE backport

See merge request gitlab-org/gitlab-ce!27407

Add endpoint to delete/uninstall a cluster application

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

Store Let's Encrypt account email in application settings
Also add explicit terms of service consent

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.