Link featured jobs to Slack

[hollomancer]

Feature

Why is this feature being added?

We want to ensure that any featured jobs we have on our site, are also featured in Slack as well (dependent on https://github.com/OperationCode/operationcode_frontend/issues/982 being completed)

What should your feature do?

MVP: Select three of the featured jobs at random, and post them in Slack under #job_board.

[hollomancer]

We'd need a table for job opportunities to do this.

[jjhampton]

@hollomancer There's a back-end PR in progress for this: https://github.com/OperationCode/operationcode_backend/issues/365