Provide an option to disable automatic queueing

Problem/Motivation

In some sites, it may not be desirable to automatically check for removable revisions every time a node is updated. For example, a site administrator may want to keep revisions normally, but periodically purge revisions by using drush to populate and process the queue. In that case, it would be helpful to provide a config option to disable automatic queueing.

Proposed resolution

Add a "Disable automatic queueing" option to the admin config form that disables the normal function of the node_revision_delete_node_update() hook.

User interface changes

Add a new "Disable automatic queueing" checkbox to the admin form.

Data model changes

Add a new 'disable_automatic_queueing' value to the config schema.