Improve some Phame custom domain remarkup and link behaviors

Summary:
Ref T6299. This makes more of the links point to the right places.

Not covered yet:

• Projects and subscribers don't point to the right place (this is a little tricky to fix, I think).
• [[ #anchor ]]s won't do the right thing in, uh, email, I guess, since uri.here is not set. This is also a little tricky.

Possibly we should just remove subscribers (although also kind of tricky).

Test Plan: On a custom-domain blog, observed that fewer things were broken.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6299

Differential Revision: https://secure.phabricator.com/D16007