code: function flattenEntries(archives) {
      return archives.reduce(function _reduceArchiveEntries(items, archive) {
          return [
              ...items,
              ...getAllEntries(archive.getGroups()).map(function _expandEntry(entry) {
                  return {
                      entry,
                      archive
                  };
              })
          ];
      }, []);
  }
docs: Flatten entries into a searchable structure
  @param {Array.<Archive>} archives An array of archives
  @returns {Array.<EntrySearchInfo>} An array of searchable objects