Configure carrierwave temp dir

Add explicit carrierwave-tmp dir

This directory will be replaced by a symlink to an NFS mounted folder in
production. By default carrierwave uses public/uploads/tmp which was not
explicitly configured.

See also:
https://github.gds/gds/alphagov-deployment/commit/84f0b4167f24bd275162f7de8feb5c8a197db30c

Ticket:
https://www.pivotaltracker.com/story/show/54371498