[#670] Don't delete keywords on IATI import

The keywords for a project should not be deleted when updating a project
using the API IATI import

This might need to be revisited if/when keywords becomme part of the
import, but for now they're managed solely through the admin.
Merge pull request #677 from akvo/feature/670_keyword_management

[#670] Don't delete keywords on IATI import