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 secure.phabricator.com through CloudFlare (almost; waiting for DNS).

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: chad, epriestley

Maniphest Tasks: T2382, T2380

Differential Revision: https://secure.phabricator.com/D11649