Add permission page.edit_head

The page check "function == p.NORM and variant == Text.HEAD" has been
introduced in 40dd3808a9a963671a7b3582dc7bdc9f03fb12ec, disabled in
111b5522f7692df353af0ac44ce6c387a7c886b9 and reenabled in
8de31ada30bcdaa23428268b14c27a1ba0988016.

My guess why it's here in the first place is to only allow instance
admins (or higher) to edit original versions.

In current use cases however it is desired to allow changing of original
versions to other groups (i.e. moderators). Therefore this precise case
is now covered with a separate permission "page.edit_head", assigned by
default to moderators and higher.

For the general mess concerning lib.auth.page and lib.auth.norm see
#572.