Fix return_url generation for MP pages

becuase there's redirect magic etc going on we should use
the canonical MP URL to generate the return url
fixup! Fix return_url generation for MP pages
Add mini survey to list of debates for a day
Add survey question across top of debates page

Nowhere else sensible to put
move mini survey about link to save vertical space
allow user to skip current mini survey question

Sets the same cookie as answering sets so we can show them the
another question