deposit: assets import clean-up

* Changes js and css import to use url_for.  (closes #1817)

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>
Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Reviewed-by: Javier Martin Montull <javier.martin.montull@cern.ch>