Track in database which files/messages/replies are decrypted

[redshiftzero]

Part of #231.

Right now we track whether or not a submission is downloaded in the database (in a column not surprisingly called is_downloaded). We should also track which files were successfully decrypted via a column called is_decrypted. This should go on both the Submission and Reply tables.

This is needed for the following situation:

1. User starts download of large file
2. Download finishes
3. Decryption of download fails for some reason

At this point, we don't really want to have to download the entire file again to attempt a decryption, so we should save all files that successfully download as such locally, and then mark whether or not decryption completed (what to do if decryption fails is not a part of this ticket).

[heartsucker]

Closed by #262.