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
Here is the code related to clean the ingest expired content so I think that the code related to clean expired sessions should be moved in other side of the application like 'auth' app.