on-the-fly user management

[darrenwee]

Currently, adding a new user to the mailing list in authorized.py is a slightly painful, manual process that also requires restarting the bot which also clears the announcement list as it is currently volatile.

Investigate a way for an admin to add a new user to the mailing list through a Telegram command, e.g. /admin add chat_id user_name and have it remain persistent. Related to #3.