R1:6a5369b173d5

Add an extensible "SiteSource" for configuration

Summary:
Fixes T2792. This adds a pluggable configuration layer between all the stuff on disk (local/file) and the runtime configurable stuff (database).

An install can subclass this source and:

- For Phacility, query a remote service (like Almanac) to retrieve hostname-based configuration, allowing one install to serve multiple instances.
- Maybe for Phacility, query a remote service (like Phlux) to retrieve sitevar-like configuration (e.g., put…