deposit: datetime picker library modification

* Changes the datetime picker library from the jQuery UI to
  bootstrap-datetimepicker.

* Increases the consistency with the Bootstrap design.

* Goes with https://github.com/inspirehep/invenio/pull/116.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Co-authored-by: Georgios Kokosioulis <georgios.kokosioulis@cern.ch>