Add a way to ask for volunteers on slack

[madcampos]

From @Zen-cronic's comment:

Hey, this is a great initiative, and I'd love to share some thoughts. Even though I'm pretty new to Tjs as a volunteer, I've noticed a pattern in how we handle volunteer sign-ups:

The organizers post the event name and details.
Including the volunteer roles and how many spots are open.
Folks comment on the thread, claiming their roles.
A few days before the event, the organizers update the thread or reply with a list of who's doing what.
The organizer has to go through the thread, count up who's signed up for which role, and then post an updated list before each event. Every event post in the volunteer channel follows this structure. Which means we could automate the whole process!

PS. I believe the JSON schema from torontojs/proposals#2 will be integrated into this for the events