Create direct submission function

[jchate6]

Users may want to automate message submission.
This will require some infrastructure and documentation:

• Build direct submission method
• Update README describing how to incorporate this into custom code.

Incorporate something like the following to publisher.py

def publish_to_hermes(message_info, datums=None, targets=None, *, user=None, **kwargs):
    """POST a fully-assembled HERMES message to ``/api/v0/submit_message/``.
    """
    if targets is None:
        targets = Target.objects.none()
    if datums is None:
        datums = []

    creds = resolve_hermes_credentials(user)
    if not creds.get('api_key'):
        return {'message': (
            'No HERMES API key available. Configure either per-user credentials on '
            "the user's HermesProfile page, or TOM-wide credentials at "
            "settings.HERMES_CONFIGURATION['HERMES_API_TOKEN']."
        )}
    if not creds.get('base_url'):
        return {'message': (
            'No HERMES BASE_URL configured. Set settings.HERMES_CONFIGURATION["HERMES_BASE_URL"].'
        )}

    stream_base_url = creds['base_url']
    submit_url = stream_base_url + 'api/v0/submit_message/'
    headers = {'Authorization': f"Token {creds['api_key']}"}

    # Build the HERMES-schema JSON body from the TOM models.
    try:
        message = create_hermes_message(message_info, datums, targets, **kwargs)
    except HermesMessageException as e:
        return {'message': 'ERROR: ' + str(e)}

    # Submit. If the POST or the status check raises, surface the HTTP
    # response to the caller so they can inspect it; don't crash the
    # request-handling view.
    response = None
    try:
        response = requests.post(url=submit_url, json=message, headers=headers)
        response.raise_for_status()
    except Exception as ex:
        logger.error(repr(ex))
        if response is not None:
            logger.error(response.content)
        return response if response is not None else {'message': f'ERROR: {ex!r}'}

    # Log a one-line summary of the successful publish. Without this the
    # only operator-visible signal in the runserver log is the bare
    # ``POST /share/ ... 302`` from Django's request log; the
    # HERMES-assigned uuid (the message id we publish under) is otherwise
    # invisible to the operator.
    message_uuid = response.json().get('uuid', '<no-uuid>')
    logger.info(
        f'publish_to_hermes: published topic={message_info.topic} '
        f'message_uuid={message_uuid}'
    )
    return response