Add support for generating mbox files

Add support for generating mbox files

This consists of two parts:

1. A script that can generate the montly mbox files like we get from
   majordomo, for non-majordomo lists. It is not an exact fit for the
   way things were in majordomo, since we still to thread-joining
   between lists (as a fundamental result of the datamodel used we simply
   do not track individual messages per list).

2. The ability to download individual threads as mbox files. This allows
   the use of a local MUA to use them, and can also serve as a good way
   to "join" a discussion in an existing thread by getting the correct
   reply headers, as long as the user has a MUA capable of dealing with
   mbox files.
   The mbox URLs are protected with the same password as the raw message
   downloads, as they also contain unfiltered email addresses.