Make forums readable to registered users by default

This has caused so many users trouble, it's about time this finally got fixed.

In the create forum screen in the ACP we already have copy forum permissions. However this will not help new users who deleted the test forum and are now setting up their new forum.

Instead select should contain an option to "Grant standard access permissions" (checked by default) or to "Grant no permissions to anyone". To make this work, the code would first check if the regular phpBB roles (Standard Access, Read Only Access) are present. If they aren't the option to grant standard access permissions will not be available (however this will probably only happen on boards with rather skilled administrators). If the user selected grant standard access permissions, then it will assign Standard Access to Registered Users and Read Only Access to guests. We could probably extend this to include proper moderator permissions for global moderators.