R1:f96b2583023e

Restore the "alternate file domain" setup warning and provide CDN instructions

Summary:
Fixes T2380. Fixes T2382. Users should really configure this, but when we had a warning before a lot of users had trouble with it.

- Tout performance benefits.
- Document easy setup via CDN.
- We have an "Ignore" button now for users who really don't care.

Test Plan:
- Set up `admin.phacility.com` through AWS CloudFront (need a few changes to handle instances to put it on the cluster in general).
- Set up…