delete_orphaned_plugins command

Improves the cms list plugins command, and adds a new command to delete orphaned plugins.

Orphaned plugins are ones that exist in the CMSPlugins table, but:

* have a plugin_type that is no longer even installed
* have no corresponding saved instance in the table for that specific plugin

The second type is easy to create inadvertently: Insert a plugin in the admin, but don't save it. One of my databases had literally hundreds of these.

Includes tests and docs.