Adding a description to page_types

Addresses: https://github.com/pyrocms/pyrocms/issues/2278

Includes small changes in the controller, model and views. Also
includes an updated table structure in details.php, as well as a
migration file to add the description field to existing installations.
Missed a line

Missed a line earlier during copying edits to this branch
Merge pull request #2297 from Solinx/2.2/dev/page_type_description

Adding a description to page_types