deposit: datetime picker library modification

* Changes the datetime picker library from the jQuery UI to https://github.com/Eonasdan/bootstrap-datetimepicker.

* Increases the consistency with the Bootstrap design.
In order for the assets to work in ``production mode`` you will need to duplicate this in https://github.com/inveniosoftware/invenio/blob/pu/invenio/base/static/js/build.js