R1:189600e4116e

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.