Allow override through view permissions

Using "View revisions by content type" currently makes the "View revisions" permission obsolete. However it can be tedious to set permissions for each content type if a role automatically should have view revision permission for all content types.

This patch uses the general "view revisions" permission as an override ie. if a role has the "view revisions" permission she can view revisions for all content types no matter how View revisions by content type permissions are set.