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/models/user.rb

@@ -91,6 +91,10 @@ class User < ApplicationRecord
     settings.auto_play_gif
   end
 
+  def setting_system_font_ui
+    settings.system_font_ui
+  end
+
   def activate_session(request)
     session_activations.activate(session_id: SecureRandom.hex,
                                  user_agent: request.user_agent,