Remove international development fund categories

International development funds are moving from whitehall to
specialist-publisher/specialist-frontend. Therefore we want to remove
them from collections browse and redirect the categories to the new
international development fund finder.
Nullify the correct mainstream category field

Sometimes the category we're removing isn't the primary category but one
of the other categories. Handle that case.
Notify the router API to add redirects
Merge pull request #1698 from alphagov/remove-international-development-fund-categories

Remove international development fund categories