This is a very old commit for #106, which has been on hiatus for a few
years. It was significantly modified by tabbott to:
* Improve coding style and variable names
* Update mypy annotations style
* Clean up the testing logic
* Update for API changes elsewhere in our system
But the actual runtime code is essentially unmodified from the
original work by Kirill.
It contains basic support for archiving Messages, UserMessages, and
Attachments with a nice test suite. It's still not usable in
production (e.g. it will probably break Reactions, SubMessages, etc.),
but upcoming commits will address that.
K.Kanakhin