add "activeDate" to user

- [x] add a column to `User` domain
- [x] find solution for date pickers for angularjs apps
- [ ] make it a date selector for new/edit form
- [ ] allow search on it with "active date between ____ and ____"

use http://angular-ui.github.io/bootstrap/#/datepicker and http://plnkr.co/edit/WtiLvfCsTR4sr0140ke0?p=preview