[#799] Updated IATI activity ID generation

New rules for generating the IATI activity ID:
1. Look whether the activity ID is filled in for the partnership of the support partner
2. If not, generate the activity ID based on the organisation ID + the Akvo project ID

Also, the related activity ID's are added as such, either from other partnerships or from the 'related activity id' field on partnership level.
Merge pull request #818 from akvo/feature/799_iati_activity_id

[#799] Updated IATI activity ID generation