typo
Allow deleting of newest text version

It is now also possible to delete the newest version of a text (page or
proposal description), if an older version exists.
Allow deleting of comment revisions

In addition to bed54e4a44c38e61f32eabdb32879f8751a64482, this commit
permits global admins to delete comment revisions from history.

This feature is restricted to global admins, as the revision objects are
deleted from the database itself, and not only marked as deleted.