Integrated tokens#show with sentences#show.

This removes the tokens#show view and expands sentences#show to give the
same information. tokens#edit is still around for editing this data, and
it redirects to sentences#show on submission.
Return objects from users.assigned_sentences in textual order.
Removed versioning restriction on redcarpet gem.
Merged the attributions page with the README file.
Removed versioning restriction on high_voltage gem.
Removed pages controller and routes.

This removes the pages controller and all associated routes, including
the route for export files. The high_voltage gem supplies its own
default routes, which are capable of serving our static Markdown files,
and the export files now all live in the public directory anyway.
Fix: update morphology_tag instead of morphology in tokens#edit.