change "added" to "published" for documents.

fix #44
Make maximum string lengths more generous.

This won't have any database-level effects because sqlite doesn't care about max string lengths anyway. Otherwise we'd need a migration.

It does stop sqlalchemy from complaining about it though, and fixes #40.
remove dead code

this model never found a use
Fix download endpoint for newly uploaded documents

Had forgotten to put 'documents' in the filesystem path.

fixes #39