clean up for nmweb/btsmail

[Daniel Kahn Gillmor]

The patches that follow are some proposed cleanups for
contrib/notmuch-web (aka nmweb, aka btsmail), which was originally
proposed by Brian Sniffen.

These are pretty mechanical changes -- nothing fancy added or removed,
just trying to make the code a little more maintainable and moving it
to python3.

You can also find this on the "nmweb" branch at
https://gitlab.com/dkg/notmuch

Regards,

        --dkg