feature: serialization of notifications into objects and their filtering
Closes #39
This MR should introduce all of the functionality needed for DjangoLDP#332, in combination with the MR in DjangoLDP-JobOffers. The design is described by the update to the documentation (README) via this MR
Note that whilst the object in the request may be serialized, when the notification is saved it is saved with only the JobOffer urlid as before. As such this MR shouldn't introduce any breaking changes
I added some limited unit tests, the most useful ones are in DjangoLDP-JobOffer. Needs to be tested on staging