Handle student & coach event emailing as separate

[gnclmorais]

Send them both in the background, separately. This should help with the timeouts we have sometimes on Heroku, I hope. Thoughts @matyikriszta? Do you think we could spin up a review-app of this on Heroku or something, and trigger test emails?

In any case, I revisited https://github.com/codebar/planner/pull/1750 and got it back to shape. ✨ That one should definitely help in sending less emails.

[matyikriszta]

@gnclmorais we currently can't set up Review apps, we'd never set that feature up on Heroku and in the app and since we are trying to move away from Heroku I do not want to invest time in it. We could potentially test on staging, I do wonder if we have enough seed data to meaningfully test this feature, we'd have to have thousands of members subscribed to a chapter to really be able to test it. I think this should help already, one other thing we could do is set the coach emails to deliver_later. Not sure how this works together with the handle_asynchronously method. Coach spots do not fill up as fast as student spots so I think it's not a big issue if everyone doesn't get the email at about the same time.