Implement granular and flexible subscriptions and notifications

[kytrinyx]

The goal here is to let people know that there's interesting new stuff, which would likely mean that they participate more frequently and for a longer amount of time, which means that they'd probably get a lot more out of Exercism (both in terms of learning and enjoyment).

This would mean integrating with a proper email platform.

Some examples of things people might want to subscribe to:

• new exercises to solve in a language track
• new solutions in a language track
• new solutions on a particular exercise in a language track
• new solutions by a particular user
• ...

We might also consider various types of digests of recent/interesting activity.

[brandondees]

FWIW I'll note that not getting external notifications is the main thing keeping me from being much more active with Exercism. I'd love to be able to get a digest email of team activity, and immediate emails corresponding to the existing site notifications. Optionally it might be nice to have a service worker and browser-based toast/popup notifications as well. That's all just for improving engagement and responsiveness for existing activities on the platform, but I would also love to see some additional nudges periodically to get back into regular practice with new exercises. The typical "we haven't seen you in a while" and "looks like you started but didn't finish X, need some help?" type things that are common for customer retention purposes in various places.

[kytrinyx]

@brandondees Thanks! Yeah we're going to be rethinking notifications from scratch as part of our design reboot #113

[nilbus]

@kytrinyx in exercism/exercism#2487:

I removed all the email functionality. It was an unconfigurable firehose.

Do you remember roughly what the monthly volume was? There are various services that offer up to 100,000 or 150,000 messages per month (small firehose? 😉 ) for free. ElasticEmail offers the most but would benefit from porting a Ruby client. This could be a useful reference when it's time to choose an email provider.

[kytrinyx]

@nilbus I don't recall what the monthly volume was, but the cost was around $80/month. ElasticEmail looks very reasonable.

[nilbus]

​Based on current sendgrid pricing, that could have been 150–200k emails/month. With more granular emails and digests, we might have lower volume. Exercism has also grown in the last 1.5 years.

[kytrinyx]

We're tracking this separately for the nextercism prototype. I'm going to go ahead and close this.