Serve docs via public/private symlinks, instead of built doc paths.

[ericholscher]

This uses the new [private|public]_symlink roots to serve files from Python.

The goal is to reduce the mass of complexity around the logic we have for serving files in Python, and keep all of that in one simple place in the filesystem around symlinks, both for public & private serving, from nginx or Python.

This changes:

• Breaks core/urls and core/views into their own directories.
• Removes support for ssl.$PRODUCTION_DOMAIN -- as this was old and unused.
• *.dev.readthedocs.io as a local domain for testing subdomains that points at 127.0.0.1 in public DNS
• Adds SERVE_DOCS as a setting to determine whether to serve public, private docs