Move the file `description` column to file_managed table.

When a file is attached to a node via a field, some handy features appear. A description field, for example, that allows users to hide the ugly file name and instead link some helpful description text.

Unfortunately, when building a view of all files, this helpful description text is lost, since it's not associated with the file, and instead is associated only with the node.

As we move file_entity into core I propose we move the file_description column from the field table to the file_managed table. Or, perhaps this could be our first default field on the file entity!

Either way, the end result should enable a user to build a view of PDFs that don't show the ugly file name but instead the pretty text that was provided for the link to the file, without needing to add a join to a field data table.