Add Footnote API endpoint
Enable footnote description updating
Update footnote primary key order

It makes sense to compose it of footnote_type_id + footnote_id. Footnote
type id is indexed 2 char field and is shorter than footnote_id so it's
faster to query.
Merge pull request #140 from alphagov/58794800-edit-national-footnote-in-place

58794800 edit national footnote in place