Modularize Aphront "sites"

Summary:
Fixes T5702. The path here is long and windy:

• I want to move blog.phacility.com to the new secure host.
• That host has security.require-https set, which I want to keep set (before, this was handled in a sort of hacky way at the nginx/preamble level, but I've cleaned up everything else now).
• Currently, that setting forces blogs to HTTPS too, which won't work.
• To let blogs be individually configurable, we need to either modularize site config or make things hackier.
• Modularize rather than increasing hackiness.
• Also add a little "modules" panel in Config. See T6859. This feels like a reasonable middle ground between putting this stuff in Applications and burying it in bin/somewhere.

Test Plan:

• Visited normal site.
• Visited phame on-domain site.
• Visited phame off-domain site.
• Viewed static resources.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5702

Differential Revision: https://secure.phabricator.com/D13474