Use eternicode/bootstrap-datepicker instead of outdated Twitter Bootstrap 1.x datepicker

This changes the datepicker from the outdated Twitter Bootstrap 1.x to the managed eternicode/bootstrap-datepicker one.  Brings benefits such as documentation and enhanced feature set like specifying format via  `data-date-format` html attribute on the input field.  

Note, this will change the default datetime format from YYYY-MM-DD to MM/DD/YYYY, which shouldn't affect anything, but if needed, we can change the default in `form.js` to the original format.

Also, I omitted the [locales directory](https://github.com/eternicode/bootstrap-datepicker/tree/master/js/locales) to reduce inflating the size of flask-admin.  A user can download and source the needed locale in their templates.