python-mysql-connector refactor

Read 538bd01 for highlights.

Deployment changes:

* config.yaml
* you need ``proxy_set_header Host $host;`` in same block as ``proxy_pass`` in your nginx config for sane ``url_for`` behavior
* schema changed; you can ``mysql urdb < schema.sql`` or similar

Tested exclusively in python3; some types may or may not have compatible/sane semantics in python2 (bytesarray <-> bytes in particular).