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

spec/lib/user_settings_decorator_spec.rb

@@ -48,5 +48,12 @@ describe UserSettingsDecorator do
       settings.update(values)
       expect(user.settings['auto_play_gif']).to eq false
     end
+    
+    it 'updates the user settings value for system font in UI' do
+      values = { 'setting_system_font_ui' => '0' }
+
+      settings.update(values)
+      expect(user.settings['system_font_ui']).to eq false
+    end
   end
 end

spec/models/user_spec.rb

@@ -184,6 +184,14 @@ RSpec.describe User, type: :model do
       expect(user.setting_auto_play_gif).to eq false
     end
   end
+  
+  describe '#setting_system_font_ui' do
+    it 'returns system font ui setting' do
+      user = Fabricate(:user)
+      user.settings[:system_font_ui] = false
+      expect(user.setting_system_font_ui).to eq false
+    end
+  end
 
   describe '#setting_boost_modal' do
     it 'returns boost modal setting' do