Add composite GenericForeignKey support

[Csirmaz Bendegúz]

This is a follow up to #373 (CompositePrimaryKey).

Proposal:

My proposal is to implement GenericForeignKey support with JSON.

1. object_id is a CharField (or TextField)
2. CompositePrimaryKey is stored as a JSON array in object_id
3. JOINs can be achieved with JSON functions (varies per db backend)

If anyone is interested, I have a proof of concept available on my ​GitHub (it's an out of sync draft right now, I'll submit a proper PR once composite primary keys are merged).

Risks:

1. What if someone is using a JSON array as the primary key (but it's not a composite primary key)?

Before deserializing the JSON array, we need to check if the content type has a composite primary key or not.

2. What if the db backend doesn't support JSON functions?

All supported databases support JSON functions.

Notes:

Django admin's LogEntry has its own implementation of "generic foreign keys".
The approach we take with GenericForeignKey should also apply to LogEntry.

If someone has a better proposal or feedback on my proposal I would be happy to hear it.

[Natalia Bidart]

Thank you Ben for creating this ticket. Accepting as described in ticket:373#comment:204.

[Csirmaz Bendegúz]

Thanks Natalia!