Move promotion email templates into this repo

[jewel-andraia]

Before event: https://gist.githubusercontent.com/billyroh/44f4ceafbaea78b1c0943251eb7346fb/raw/d29233283c36ccd6d2c3798435df6d71daddc69b/reminder.md

After event: https://gist.githubusercontent.com/billyroh/bf261fc0dc152631197279ca02a1beb1/raw/5dccb9626ace3f922acec1a0bd4d374ba60db34f/thank-you.md

[jewel-andraia]

Update the "want to speak?" issues link to include filtering by ?label=cfp