Allow inviting a user to channel via email address before they actually join Slack

It feels like it's a common follow-up when running a session (for example, Dolphin Tank or Civic Tech 101) that people aren't yet on Slack, but we want to pull them into a room when they _do_ join.

Right now, the solution is either to:
1. Take time to get everyone invited, and wait on them to check their email and join in-session, then invite them into the right channel
2. Follow-up after and hope that it's easy to find them, and that they follow-through on the sign-up confirmation after submitting the invite form at https://link.civictech.ca/slack

Would be nice if we could run
`@toby invite maria.hernandez@example.com ola.nordmann@example.com`

And then the bot would watch for [`team_join` events](https://api.slack.com/events/team_join) and check for that email address, then invite automatically if required

Start by locating the command entry point for `@toby invite` and the handler for Slack `team_join` events. Implement the email-based pending invitation flow so matching new members are invited to the requested channel, then verify that multiple email addresses and already-invited users behave as expected.

Written by the indexing model from the issue text.