Web based self-invite

[bruntonspall]

The invite functionality could be triggered from a webpage, which asks for the users email address and if it matches the whitelist, then send the invite.

[bruntonspall]

At the moment, you can directly message the bot on slack and say "invite spam@brunton-spall.co.uk" and it will either send an invite, or in this case, tell you that brunton-spall.co.uk is not whitelisted for an email invite and refuse.

The code at the moment hosts 2 very very simple pages, /users and /channels

The code to serve those is currently awful, and looks like this: https://github.com/bruntonspall/xgovslackbot/blob/master/app/slack_bot.js#L59

Anyway. that whole setup webserver should be migrated out to a standalone module. I'd double check, but I think botkit includes express, so we could use the features of express to setup routes properly and probably templates.
Then we create a route that accepts a GET on /, and points people to the signup page, a GET on /signup that shows an email address form, and a POST handler that accepts the response, does some input validation, and then passes the email to the invite function. The rest should just work as expected I think.