Change privacy policy to be rendered in web UI, add REST API (#19310)

Gemfile

Gemfile.lock

...ntrollers/api/v1/instances/privacy_policies_controller.rb

app/controllers/privacy_controller.rb

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

...javascript/mastodon/features/ui/components/link_footer.js

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

app/javascript/mastodon/features/ui/util/async-components.js

app/javascript/styles/mastodon/components.scss

app/models/privacy_policy.rb

app/serializers/rest/privacy_policy_serializer.rb

app/views/privacy/show.html.haml

config/i18n-tasks.yml

config/locales/en.yml

config/routes.rb