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:
https://github.gds/gds/alphagov-deployment/commit/84f0b4167f24bd275162f7de8feb5c8a197db30c
Avoid losing temp dir setting in dev mode

The use of a class variable plays badly with rails automatic reloading
in development mode. The variable setting would be lost if the
bulk_upload.rb file was reloaded. Using a constant avoids this
possibility.
Merge pull request #785 from alphagov/configure-carrierwave-temp-dir

Configure carrierwave temp dir

https://www.pivotaltracker.com/story/show/54371498