Provide a stable URI for getting raw paste content

Summary: Fixes T9312. This is a bit fluff, but does simplify the view controller slightly and seems reasonable/useful in general.

Test Plan: Clicked "View Raw File" on a paste, got redirected to the raw file via a stable URI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9312

Differential Revision: https://secure.phabricator.com/D14167