Add trends UI with admin and user settings (#11502)

app/controllers/settings/preferences_controller.rb

app/javascript/mastodon/actions/trends.js

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

...n/features/getting_started/containers/trends_container.js

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

...cript/mastodon/features/ui/components/navigation_panel.js

app/javascript/mastodon/initial_state.js

app/javascript/mastodon/reducers/index.js

app/javascript/mastodon/reducers/settings.js

app/javascript/mastodon/reducers/trends.js

app/javascript/styles/mastodon/components.scss

app/lib/user_settings_decorator.rb

app/models/form/admin_settings.rb

app/models/trending_tags.rb

app/models/user.rb

app/serializers/initial_state_serializer.rb

app/views/admin/settings/edit.html.haml

app/views/settings/preferences/appearance/show.html.haml

config/locales/en.yml

config/locales/simple_form.en.yml