Prefix messages with list name

Major refactor by Daniel Carter to talk to the new lists.cam.ac.uk Sympa service (Mailman has gone).  Thanks, Daniel!

Seemingly lists.cam.ac.uk no longer wants &#...; entity refs, and instead just wants real UTF-8

Switch from lxml to html.parser for CSRF token extraction, as bs4's lxml support on Ubuntu 16.04 throws a DeprecationWarning

Mailman 2.1.15 added a CSRF token on admin forms so include that in the request if needed. (Added bs4 dependency.)

Fix output formatting when we have two headings from Mailman

Default to Python3.  Handle non-ASCII characters in names a bit better under Python3 (and work around some Mailman brokenness).  Also slightly improve error handling and output formatting resilience in the presence of unusual HTML, e.g. from character entities.

Improve help text wording

Initial commit, based on srcf-process-mlsubqueue but substantially genericised