Add a setting allowing the use of system's default font in Web UI (#4033)

* 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

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

@@ -44,6 +44,7 @@
 
   .fields-group
     = f.input :setting_auto_play_gif, as: :boolean, wrapper: :with_label
+    = f.input :setting_system_font_ui, as: :boolean, wrapper: :with_label
 
   .actions
     = f.button :button, t('generic.save_changes'), type: :submit