Modify links on pieces/show view
Clean up links on pieces/index view
Add debug info to bottom of footer in dev env
Delete versions controller
Update Pieces create/edite with immutable versions

Update Pieces controller to handle creating and editing pieces that
retain immutable versions.
Update models to allow validating presence of related models.
Update Piece#current_version to return the latest version.
Update pieces/form to handle version data properly.