Add ability to disable shared runners

...scripts/group_settings/components/shared_runners_form.vue

app/assets/javascripts/group_settings/constants.js

...assets/javascripts/group_settings/mount_shared_runners.js

...ets/javascripts/pages/groups/settings/ci_cd/show/index.js

app/helpers/ci/runners_helper.rb

app/views/groups/runners/_index.html.haml

app/views/groups/runners/_shared_runners.html.haml

app/views/projects/runners/_shared_runners.html.haml

...iews/shared/runners/_shared_runners_description.html.haml

changelogs/unreleased/disable-shared-runners-ui.yml

doc/api/groups.md

lib/api/helpers/groups_helpers.rb

locale/gitlab.pot

spec/features/runners_spec.rb

...end/group_settings/components/shared_runners_form_spec.js

spec/helpers/ci/runners_helper_spec.rb