[#945] Moved IATI activity ID to the Project model

- Added field 'Project.iati_activity_id'
- Moved data from Partnership to Project
  - If the project has a 'sync_owner', use the IATI activity ID of the sync owner
  - Else, use the IATI activity ID of one of the support partners
  - Leave empty if all of these do not exist
- Added IATI activity ID field to Project admin
- Removed IATI activity ID field from Partnership inline
- Added IATI activity to the project main page, if available
Merge pull request #951 from akvo/945_iati_id

[#945] Moved IATI activity ID to the Project model