deposit: filter to mapper rename and cleanup

* Renames filter concept to mapper and updates documentation.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>
Reviewed-by: Javier Martin Montull <javier.martin.montull@cern.ch>
deposit: data source concept

* Separates data sources as a plugable objects
  for deposit import.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>
deposit: importer name change to task_manager

* Small changes to import related JS code.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>