Add way to add each document's `file_hash` and `pages` value to database

[anthonydb]

Upon upload, the DocumentCloud API response does not include the values for file_hash or pages, probably because those get calculated during the processing of the document and are not available when the file is dropped off.

I'd like to add a function in db.py to walk through the database of uploaded files and retrieve those values for each doc. It should include multiprocessing on supported platforms.

[anthonydb]

Closed via d71a56a

[anthonydb]

Reopening to remind myself that the update_processed_files method needs to write something to the database indicating a file is not found.