deposit: import button fix feedback

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

* Simplifies code related to multiple tasks and deferred objects.

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

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