Add ability to subscribe to mailing lists easier

[turt2live]

This is related to #1.

Currently mailing lists require manual intervention (and a postfix server) to subscribe. It would be nice if people could subscribe on their own without having to go through the server administrator.

Note: There is a gotcha to having this: Mailing lists tend to lowercase the email. So an email for the room !AAAbbbbCCC:domain.com could come back to aaabbbbcc_domain.com@email.t2bot.io instead of AAAbbbbCCC_domain.com@email.t2bot.io - a custom mapping can be set up to combat this, however. The bot should check for duplicate targets for the rare case of room IDs colliding when using lowercase versions.