Fix broken JS dependency in header_footer_only

Selectable JS is included in core.js, but assumes the FET+dep is already
in scope, which is setup for application, but not header_footer_only.

Introduced in https://github.com/alphagov/static/pull/449/
Merge pull request #458 from alphagov/fix-js-selectable-dependency

Fix broken JS dependency in header_footer_only