* Enable updating additional account information from user preferences via rest api
Resolves #6553

* Pacify rubocop

* Decoerce incoming settings in UserSettingsDecorator

* Create user preferences hash directly from incoming credentials instead of going through ActionController::Parameters

* Clean up user preferences update

* Use ActiveModel::Type::Boolean instead of manually checking stringified number equivalence

* Add unfollow modal

* unfollowing someone

* remove unnecessary prop

* add a system_font_ui setting on the server

* Plug the system_font_ui on the front-end

* add EN/FR locales for the new setting

* put Roboto after all other fonts

* remove trailing whitespace so CodeClimate is happy

* fix user_spec.rb

* correctly write user_spect this time

* slightly better way of adding the classes

* add comments to the system-font stack for clarification

* use .system-font for the class instead

* don't use multiple lines for comments

* remove trailing whitespace

* use the classnames module for consistency

* use `mastodon-font-sans-serif` instead of Roboto directly

* Set delete_modal preference to true by default
* Does not show confirmation modal if delete_modal is false
* Add ja translation for preference setting page

* Add missing fields group on preferences page

* Clean up settings/preferences controller

* Extract a UserSettingsDecorator