Adding in CNAME for ZenDesk

[jadudm]

I believe this change should create a CNAME record for the FAC ZenDesk instance @ support.fac.gov. The documentation is here:

https://support.zendesk.com/hc/en-us/articles/4408838571930-Host-mapping-Changing-the-URL-of-your-help-center

Forked for PR against 18F/dns per slack conversation w/ JJ.

• [ ] This is a new public-facing site (if so, please follow the additional instructions below)
  • [ ] Provide context
  • [ ] Assign to @18F/osc for review
  • [ ] TTS Digital Council's new site review process completed
  • [ ] Update the inventory with new site information