Re-work identifier to avoid a custom SHA1 identifier.

[richardkmichael]

We compute a SHA1 as the identifier because we can't use a filename (.find_by_filename!()), because the filename won't necessarily be unique.

CarrierWave uses a before_save callback to invoke write_identifier: https://github.com/jnicklas/carrierwave/blob/master/lib/carrierwave/orm/activerecord.rb#L29

Because it's a before_save, the file has not been saved and we can't use the carrier_wave_files table ID as the identifier.

(What about giving the file to the storage engine, and receiving an identifier in return? E.g. Call store! and write_identifier in the before_save callback; or call write_identifier in an after_save callback?)

Or, use the filename as the identifier and change retrieval to use the associated model as well (e.g. identifier is effectively filename + model). However, this requires storing the model name and ID in the carrier_wave_files table (otherwise, we won't have any model information to join on).