Add subscription functionality

[jonathan-d-zhang]

Subscribing?

A user may want to be notified when a new version of a package has been scanned or may want to be notified when their queued package been scanned. This feature should address both these situations.

What do?

Add a set of commands used to control subscription status for a user. These should be pretty basic crud operations: we would have "create subscription", "delete subscription".

The queue command should be modified to subscribe the user that queued the package for a single version, i.e., they should only be notified once.

How do?

While the bot is already running through scans and posting them on Discord, it will also compare it with subscriptions and ping the necessary people

Blocked?

Blocked by https://github.com/vipyrsec/notifier/issues/2