Fixes #8189

rack-cors being called before the application router, it does not follow
the redirection, and we need a separate rule for /users/:username.

Right now, this includes three endpoints: host-meta, webfinger, and change-password.

host-meta and webfinger are publicly available and do not use any authentication. Nothing bad can be done by accessing them in a user's browser.

change-password being CORS-enabled will only reveal the URL it redirects to (which is /auth/edit) but not anything about the actual /auth/edit page, because it does not have CORS enabled.

The documentation for hosting an instance on a different domain should also be updated to point out that Access-Control-Allow-Origin: * should be set at a minimum for the /.well-known/host-meta redirect to allow browser-based non-proxied instance discovery.