Use ObjectIdField instead of CharField(24)

[WaVEV]

Add ObjectIdField.
The only concern I have is: is convenient to change all charfield(24) to objectId? or keeping in that way for the other test is ok?

This is needed to fix 161

[Jibola]

The only concern I have is: is convenient to change all charfield(24) to objectId? or keeping in that way for the other test is ok?

I loosely remember there being issues around string comparison before? I'd say try replacing everything with ObjectId and see if that now keeps tests passing.

[timgraham]

While users could use object_id = ObjectIdField() for GenericRelation, this isn't cross-database compatible (for example, the admin uses LogEntry.object_id = TextField()), and I think we wouldn't promote use of GenericRelation anyway since it involves $lookup.

[timgraham]

We decided not to make this change since it breaks other tests that store data of other types.