[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.
Merge pull request #29 from mwoodiupui/HPT-160

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