UI refresh

Getting the PopIt UI ready for live demonstration.

See tickets:
- https://github.com/mysociety/popit/issues/374 (main one)
- https://github.com/mysociety/popit/issues/375 (empty homepage)
- https://github.com/mysociety/popit/issues/376 (populated homepage)
- https://github.com/mysociety/popit/issues/381 (person page)

Not ready to merge.