Add rake task for reprocessing only meta data

The previous way of generating meta data for existing
Paperclip::Attachments was to reprocess them all.  For existing
implementations with lots of records and many styles that take time to
resize and process thumbnails that could be a lot of overkill.   This
creates a simple task that only generates the meta data for existing
attachments and their styles.
y, I always used to prefer inject as it read better to me, but my old team enforced the ruby style guide via rubocop, and hence I was retrained to use inject: https://github.com/bbatsov/ruby-style-guide#map-fine-select-reduce-size