Messaging/Notification Subscription System

[rickr]

We should be able to deliver meaningful notifications to our members.

This system should operate like so:

Upon sign up a user should be able to select areas they're interested in. These areas should be high level, eg ruby, javascript, react, angular.

When Operation Code receives an opportunity a staff member should be able to craft a message, tag one of these areas, and be able to send to everyone that is interested in said area.

The message should be delivered to relevant slack channels as well as email.