Add server banner to web app, add `GET /api/v2/instance` to REST API (#19294)

app/controllers/about_controller.rb

app/controllers/api/v1/instances_controller.rb

app/controllers/api/v2/instances_controller.rb

app/javascript/mastodon/actions/rules.js

app/javascript/mastodon/actions/server.js

app/javascript/mastodon/components/account.js

app/javascript/mastodon/components/display_name.js

app/javascript/mastodon/components/server_banner.js

app/javascript/mastodon/features/report/rules.js

...vascript/mastodon/features/ui/components/compose_panel.js

...avascript/mastodon/features/ui/components/report_modal.js

app/javascript/mastodon/features/ui/index.js

app/javascript/mastodon/initial_state.js

app/javascript/mastodon/reducers/index.js

app/javascript/mastodon/reducers/rules.js

app/javascript/mastodon/reducers/server.js

app/javascript/styles/mastodon/components.scss

app/presenters/instance_presenter.rb

app/serializers/initial_state_serializer.rb

app/serializers/manifest_serializer.rb