Add can_edit callback to content base class for custom security

[peakpg]

Imported from lighthouse. Original ticket at: http://browsermedia.lighthouseapp.com/projects/28481/tickets/327. Created by buzzware - 2010-03-21 23:11:15 UTC

To enable more selective security, a call back per content type would receive the content block in question, and return true or false to determine whether the user is able to edit that particular block. This would also determine the status of a padlock in the list of content blocks for that type.