documents: files field rename

* Renames `_files` and `files` to `_documents` and `documents` to avoid
  name collisions with current bibupload fields. (closes #1898)

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
records: fix `get_blob` to ease transition.

* With the new uploader there is a new column inside `bibfmt` to allow
  different kind of versions of the record: `master`, `DIP`, etc. but
  bibupload is not filling this column, only `format='xm'` to identify
  the master version for `marcxml`.

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>