Sort facets out properly

Don't use dict.update() on dataset_facets, as it removes the ordering.
Tweak the facet fields before indexing (before_index endpoint) so they
display nicely on the frontend, using new helpers.
Merge branch 'ckan-2.x' of github.com:okfn/ckanext-iati into ckan-2.x

Conflicts:
	ckanext/iati/helpers.py
	ckanext/iati/plugins.py
Add facets for organization page
Update publisher form

Add more text helpers and field for image_url
Add missing fields to package_item snippet. Needs styling