[Org] Onboarding welcome emails

@leowilkin is working on sending "welcome" emails to users who have just been onboarded. Here's a draft he sent me:


----

@leowilkin, go ahead and create an `Event::OnboardingMailer`. I'll then help with the scheduling of these emails.

You can get Rails to do most of the work for you by running:

```bash
bin/rails g mailer event/onboarding
```

It'll generate these files:
```
create app/mailers/event/onboarding_mailer.rb
invoke erb
create app/views/event/onboarding_mailer
create app/views/layouts/event/mailer.text.erb
create app/views/layouts/event/mailer.html.erb
```
- You can delete app/views/layouts/event/mailer.text.erb
- In app/views/layouts/event/mailer.html.erb, get rid of the HTMl shell and just put in the body of your email. Our [mailer layout](https://github.com/hackclub/bank/tree/97837b74f91e332331c90e7287f04bfaa465ce6b/app/views/layouts/mailer) will handle the HTML shell.