The imageable/documentable object is always the object the form builder
is based on; since we're already passing the form builder, we don't have
to pass the object as well.
The only exception are the poll answers. In this case, we're passing a
new answer as the object. That's OK; the same hack that we're using to
send the data to the answer URL without displaying existing attachments
causes the form to keep working the same way.
We don't need any row classes anymore because the <body> already has a
maximum width. As for columns, we only have one column in this form, so
we don't need them either. Besides, the form's parent element already
has a padding.
Since now categories are loaded in both the investment form component
and proposal form component, and their controllers are the only
"commentable" controllers using the `@categories` instance variable, we
can remove the `load_categories` call in `CommentableActions` affecting
the create and update actions.
