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

config/locales/simple_form.en.yml

@@ -38,6 +38,7 @@ en:
         setting_boost_modal: Show confirmation dialog before boosting
         setting_default_privacy: Post privacy
         setting_delete_modal: Show confirmation dialog before deleting a toot
+        setting_system_font_ui: Use system's default font
         severity: Severity
         type: Import type
         username: Username

config/locales/simple_form.fr.yml

@@ -28,6 +28,7 @@ fr:
         password: Mot de passe
         setting_boost_modal: Afficher un dialogue de confirmation avant de partager
         setting_default_privacy: Confidentialité des statuts
+        setting_system_font_ui: Utiliser la police par défaut du système
         severity: Séverité
         type: Type d'import
         username: Identifiant