db_schemas: convert 'key' to 'keysum'

- 'keysum' is the checksum key for a given file in a repository or on
  a shovelware CD on archive.org
idGamesFile: repurpose 'key' attribute for 'base_url'
idGamesFile: issue #118; added `keysum` method

- `keysum` computes a repeatable checksum given a file's base URL, path, and
  filename.  Keysums would be used in place of full URLs to refer to a
  given file in either idGames Archive or another website/shovelware CD