So uploading files via remote URLs was completely broken and
apparently nobody noticed. This commit fixes that, too.
Wouldn’t it be nice if there were a test suite!

Werkzeug uses tempfile.SpooledTemporaryFile, so we can make use of
file-like object properties. This may result in more disk writes,
but that’s probably better than eating up RAM.

I hope this fixes #84.

Will be readded after some major refactoring and modernization.

This moves preexisting blacklists to the database, and adds the
following filter types:

* IP address
* IP network
* MIME type
* User agent

In addition, IP address handling is now done with the ipaddress
module.

Fixes #85