[#121] Use "constants" for bundle field names for Partnership and
Organisation

These fields are used so many times it makes sense to put them in
identifiers to avoid misspelling
[#121] Extend functionality of IATIPartnershipResource

The IATIPartnershipResource can now handle the creation of Partnership
objects and the linking of Organisation objects either creating the
Organisations on the fly of linking to existing ones.

There is quite a lot of custom code for Cordaid right now that should
be extracted from the general IATI resources (if possible).