Reintroduces legacy forum config tab for installations still using legacy forums

Without access to this page, it becomes impossible to regenerate the
bb-config.php file. We also lose the chance to preach the gospel of the new
bbPress plugin to users of bbPress 1.x when we don't have an admin panel for
the legacy component.

Fixes #4999

Props r-a-y

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7213 cdf35c40-ae34-48e0-9cc9-0c9da1808c22