improved duration and minute selector (#2264)

* do not close modal if form is dirty
* deprecated TimesheetConfiguration
* inject timezone in form types
* cleanup usage of UserDateTimeFactory
* allow to configure increment steps for minutes
* use 15 minutes step for datetimepicker in project edit form
* use rounding rules for increments in minute select for begin and end
* allow duration in multi user and admin timesheet forms
* make dropdown values configurable

assets/sass/app.scss

@@ -27,3 +27,4 @@
 @import 'selectpicker';
 @import 'reporting';
 @import 'forms';
+@import 'modal';