(js) Calendar week days must be set to save prefs

UI/Templates/PreferencesUI/UIxPreferences.wox

@@ -311,6 +311,7 @@
                 <md-grid-list md-cols="7" md-row-height="2em" md-gutter="0.5em"
                               flex="50" flex-sm="80" flex-xs="100"
                               ng-model="app.preferences.defaults.SOGoCalendarWeekdays"
+                              ng-required="true"
                               sg-toggle-grid="sg-toggle-grid">
                   <var:foreach list="shortWeekDaysList" item="item">
                     <md-grid-tile var:value="valueForWeekDay"><var:string value="item"/></md-grid-tile>