Add 3 domains to whitelist for DFT

www.dft.gov.uk is currently supplier-redirected and already redirects
a few pages to these domains. They were added to Redirector in
https://github.com/alphagov/redirector/pull/484 - the other domains
added there are *.gov.uk subdomains so don't need to be included in
Bouncer's whitelist.
Merge pull request #36 from alphagov/whitelist_domains_for_dft

Add 3 domains to whitelist for DFT
Add DFT regex rules to replicate existing behaviour