Allow broader HTTP access to public repositories, respect nonstandard Phabricator HTTP port when generating repository URIs

Summary:
Fixes T11030. Fixes T11032.

• Allow HTTP access to "Public" repositories even if diffusion.allow-http-auth is disabled.
• If you run Phabricator on an unusual port (???) use that port as the default when generating HTTP URIs.

Test Plan:

• Faked phabricator.base-uri to an unusual port, saw repository HTTP URI generate with an unusual port.
• Disabled diffusion.allow-http-auth, confirmed that toggling view policy between "public" and "users" activated or deactivated HTTP clone URI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11030, T11032

Differential Revision: https://secure.phabricator.com/D15973