User preferences, session preferences, purge old sessions, session based locking.

SD-6833 sub-tasksSaved preferences on log-out
SD-7013 sub-tasksAs a user, I want to sign in to Superdesk multiple times simultaneously
For session expired can be defined a separated Command only for sessions expired and other command related to auth and users. You have to define a celery task as one here: https://github.com/marwoodandrew/superdesk-server/blob/master/superdesk/io/__init__.py#L31
Then you have to set the celery beat settings like here:
https://github.com/marwoodandrew/superdesk-server/blob/master/settings.py#L50