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.