Admin API: `GET /api/pleroma/admin/stats` to get status count by visibility scope

CHANGELOG.md

docs/API/admin_api.md

lib/mix/tasks/pleroma/refresh_counter_cache.ex

lib/pleroma/counter_cache.ex

lib/pleroma/stats.ex

lib/pleroma/web/admin_api/admin_api_controller.ex

lib/pleroma/web/router.ex

...epo/migrations/20200109123126_add_counter_cache_table.exs

test/stat_test.exs

test/tasks/refresh_counter_cache_test.exs

test/web/admin_api/admin_api_controller_test.exs