feature: methods to serialize and filter JobOffer notifications

Requires:

• djangoldp!230 (merged)
• djangoldp-notification!48 (merged)

An MR which should meet the users' needs behind djangoldp#332 without changing the core library or introducing breaking changes. Also added tests. Needs to be tested on staging

See updated README for how to test with djangoldp-notifications

Following the unit tests, saving a JobOffer should lead to a notification being sent to each user on the distant server with a matching skill. It should also result in a notification being sent to each user on the local server with a matching skill