[HPT-160] Replace physical page number with previous-, next-page references

Also undo a previous commit that split the composition of params.require from params.permit in Page.  That previous commit papered over the real problem, which was that the tests themselves were wrong, and broke Page.update and Page.save.
This was wrong.  :page here is not the model, but a hash of field values provided by the view *to be copied into the model*.