Chapps

- introducing the first version of the REST API; INSTALLATION
instructions will be modified to discuss and provide references
for proper, highly-available deployment of the REST API, which
it is advised be run on separate servers.

- The API is powered by [FastAPI](https://fastapi.tiangolo.com/);
detailed instructions regarding API deployment may be found in
its documentation, including how to use a reverse proxy such as
**nginx** to provide SSL.

- Database access for the API is performed using
[SQLAlchemy](https://www.sqlalchemy.org/), and in future
releases, all database accesses will be converted to using it,
for the sake of consistency and to eliminate the needless
dependency on the MariaDB client software.

- PyPI package now provides the `[API]` extras definition, to
install API prerequisites (which are not otherwise needed)

- CHANGELOG adapted to be Markdown-compatible

- improvements to setuptools install process (pip / setup.py)
which should make it possible to format correct SystemD service
description files to run the installed CHAPPS services, even
from within their venv

- fixing a problem with logging, wherein library logs were suppressed

- generally tidying logging

- updated documentation to discuss logging

- refined installation instructions

- making OQP throttling disabled by default; it needs some work

- adding log handler to send messages to the mail log

- modifying logic around user-key extraction, and adding new
settings: require_user_key (boolean), and no_user_key_response
(Postfix response string). See README for more information.

- adding some logic to guard against IndexError conditions when
calculating time-delta from the attempts list

- adding default config value for `min_delta` to
OutboundQuotaPolicy, which defaults to 5. It represents the
minimum number of seconds between attempts; faster attempts will
be refused (but reset the timer).

- correcting problem with recipient-counting wherein the
recipients memoizing routine ran afoul of __getattr__()

- correcting problem with recognizing at signs in email addresses