Add graphs and retention metrics to admin dashboard (#16829)

app/controllers/admin/dashboard_controller.rb

app/controllers/api/v1/admin/dimensions_controller.rb

app/controllers/api/v1/admin/measures_controller.rb

app/controllers/api/v1/admin/retention_controller.rb

app/controllers/api/v1/admin/trends_controller.rb

app/controllers/api/v1/instances/activity_controller.rb

app/helpers/application_helper.rb

app/javascript/mastodon/components/admin/Counter.js

app/javascript/mastodon/components/admin/Dimension.js

app/javascript/mastodon/components/admin/Retention.js

app/javascript/mastodon/components/admin/Trends.js

app/javascript/mastodon/components/hashtag.js

app/javascript/mastodon/components/skeleton.js

app/javascript/mastodon/containers/admin_component.js

app/javascript/mastodon/containers/media_container.js

...pt/mastodon/features/compose/components/search_results.js

...pt/mastodon/features/getting_started/components/trends.js

app/javascript/mastodon/utils/numbers.js

app/javascript/packs/admin.js

app/javascript/styles/mastodon/admin.scss