As a random user, I want to be able to subscribe to job board notifications

What needs to be done?

As discussed here: #680 (closed)

Once communities are implemented, each user should be able to subscribe to offers from a particular community.

We need to define the email content.

Technical details

Are there any technical details worth mentioning?

Test cases

1. Go to ??
2. Subscribe to a community job offers
3. Check you get an email when a new offer is published
4. Retest with unsubscribing

Links & Mockups

Previous issues that might be linked: #155 (moved)

#137 (moved)