Add support for Trac 1.0 and later

[Ryan J Ollos]

It was discovered in #11008 that the plugin is not compatible with Trac 1.0. There are several issues:

• The XPath selector need to be modified to account for changes in the ticket template.
• cnum is no longer passed as a request arg. The database schema may need to be changed to link the private_comment table through a different variable. Alternatively, it might be possible to add a private field to the ticket_change table to avoid the need for a second table.