Add default setting for first day of the week preference

app/helpers/preferences_helper.rb

@@ -43,8 +43,13 @@ module PreferencesHelper
     ]
   end
 
+  def default_first_day_of_week
+    Date::DAYNAMES[Gitlab::CurrentSettings.default_first_day_of_week]
+  end
+
   def first_day_of_week_choices
     [
+      ["System Default (#{default_first_day_of_week})", nil],
       ['Sunday', 0],
       ['Monday', 1]
     ]

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

@@ -75,7 +75,7 @@
     .form-group
       = f.label :first_day_of_week, class: 'label-bold' do
         First day of the week
-      = f.select :first_day_of_week, options_for_select(first_day_of_week_choices, @user.first_day_of_week || Gitlab::CurrentSettings.default_first_day_of_week), {}, class: 'form-control'
+      = f.select :first_day_of_week, first_day_of_week_choices, {}, class: 'form-control'
       .form-text.text-muted
         Choose on what day the week should start.
     .form-group

doc/user/profile/preferences.md

@@ -94,4 +94,5 @@ This following setting allows you to customize calendar layouts.
 
 ### First day of the week
 
-You can choose between **Sunday** and **Monday** for the first day of the week. This will be used for all calendar views and datepickers.
+You can choose between **Default**, **Sunday** and **Monday** for the first day of the week. If you select **Default**, the system-wide default setting will be used.
+This preference will be used for all calendar views and datepickers.