Notification delivery with PubSub

[w0rd-driven]

• [x] Create a Beatseek.Notifications.Delivery module
• [x] Add a deliver function with pattern matching that handles in-app notifications with abstractions for Email and Slack/Discord.
• [x] In-app delivery adds a notifications record then pushes to PubSub events.
• [ ] Next: Email requires a notifications record to transform and send email.
• [ ] Next: Slack and Discord requires a notifications record to transform and send a Slack message.

We'll focus on in-app messaging first. PubSub events should emit the record to add to the notifications page as well as an increment. Marking a record as read/seen should emit a decrement.

[w0rd-driven]

I should be able to call functions in iex and watch things update in real-time.

[w0rd-driven]

The stubbing proved as useful as I thought it would be. Now I need to clean up the implementation to inspect release dates or to send generic app notifications like when a scan completed.

[w0rd-driven]

This is done. We can address email and Slack in subsequent issues.