Email Notifications

[dmbf29]

We had a lot of user drop off in the knockout stages when people didn't know to make more predictions.

I added emails to the daily job that Heroku schedules.
It will send an email for every match that you don't have a prediction for.
(assuming that you have notifications turned on and have previously made picks in this competition)
I also created an email model so we are careful not to repeat emails.

The style isn't perfect but better than nothing?

⚠️ I put a link to the profile page of a user, which means we need to update the user profile page to unsubscribe from there. Either that or an auto unsubscribe which would be better i think.

⚠️ This PR should have the front-end PR to work fully.