Add support for custom deprecation messages in main_prefix partial

Some working group charters have a unique deprecation message:

• https://www.eclipse.org/org/workinggroups/osbea_wg_charter.php
• https://www.eclipse.org/org/workinggroups/mobilewg.php
• https://www.eclipse.org/org/workinggroups/eclipse_edge_charter.php

We should add support for this by enabling the use of a message page param.