add a test that warns when calling ::property in cell class but not passing a model into the constructor.
allow the new trailblazer file structure where cells (or concepts in general) are fully self-contained in their own directory.
this means a cell could now have assets like this:
app/cells/comments/[comment_cell.rb]
                   cell.rb
                   views/
                         show.haml

use Cell::Base::self_contained! to enforce this new standard.