Data migration to restore alt format providers

Previously, the alternative format provider was set on the policy and
infered from there. Now, it must be explicitly set on the supporting
page. This data migration sets the alternative format provider on
those supporting pages that don't have one, based on the value of the
parent policy.
Remove redundant code

Policies have no truck with alternative format providers anymore.
Merge pull request #1097 from alphagov/61219962-restore-afp-for-supporting-pages

61219962 restore afp for supporting pages

[Fixes https://www.pivotaltracker.com/story/show/61219962]