Add a recovery system

[thomaspiller]

Add a system to keep deleted content (i.e., views, sources) for a period of 30 days to allow recovery in the event of erroneous deletion.
Deleted content will be flagged as such in the db (e.g., deleted column of timestamp type) and will no longer be considered in MapX. A daily routine will take care of removing deleted content after 30 days in the event that no recovery has been made.