Ability to customize the access denied message

Problem/Motivation

When this module is used to blacklist pages, the message given to the end-user who has been denied access is hard-coded and cannot be customized by site editors.

Proposed resolution

Patch is attached. It creates a new text area for a custom message on the module settings page. It only appears if the blacklist pages setting is turned on.

Data model changes

Adds a new variable to store the custom message.