Route policy declarations (for network routes only for now).

Allows packages such as stream and accounts to declare URL prefixes
such as /sockjs/ and /_oauth/ to be network routes.

Updates server to avoid serving app HTML on network routes.

Checks for conflict between files in public/ and network routes.  For
example, the developer might not know that /sockjs/ is reserved, and
might create a file "public/sockjs/socks-are-great.png".
Merge remote-tracking branch 'remotes/origin/pr/679' into devel. Fixes #679.