Read-only state for CKAN

[dread]

When performing maintenance on CKAN it may be necessary to make CKAN obviously read-only, telling the users and restricting access to 'edit' pages.

Example of maintenance would be switching between two parallel CKAN instances - you want to make it RO when you do the backup and until you bring up the new one.

This would also be useful for the public DGU read-only CKAN.

It would also be useful for a fail-over CKAN server that is only used in the event of problems, and no edits will be transferred back to the main server.