Add setting a always mark media as sensitive (#4136)

app/controllers/settings/preferences_controller.rb

app/javascript/mastodon/reducers/compose.js

app/lib/user_settings_decorator.rb

app/models/user.rb

app/serializers/initial_state_serializer.rb

app/serializers/rest/credential_account_serializer.rb

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

config/locales/simple_form.en.yml

spec/lib/user_settings_decorator_spec.rb