ReviewBoard currently supports attaching individual files for review,
but while some filetypes can be opened in-browser and examined, archive
files can only be downloaded. This commit adds a new review UI that
allows archives to be browsed in ReviewBoard without downloading the
file. Users can navigate the archived files, see metadata, and download
individual files.

Unit tests for get_js_model_data using empty archives, simple archives with files, and archives with nested directories.