R1:ae7324fd5b8a

Fix an anchor redirect issue with OAuth server, plus modernize the application a bit

Summary:
Ref T4593. Via HackerOne. An attacker can use the anchor reattachment, combined with the Facebook token workflow, combined with redirection on OAuth errors to capture access tokens. The attack works roughly like this:

- Create an OAuth application on Phabricator.
- Set the domain to `evil.com`.
- Grab the OAuth URI for it (something like `https://phabricator.com/oauthserver/auth/?redirect_uri=http://evil.