Modify post-review to support review-board deployment under a directory instead of the root path

Review Request #369 — Created May 4, 2008 and submitted — Latest diff uploaded

Review Board SVN (deprecated)
I deployed a test review-board server under /reviewboard by modifying SITE_ROOT but all of the API calls in post-review were being resolved by urljoin to the root even when REVIEWBOARD_URL included the directory. After making these changes, everything still resolved fine when not using a directory.
I posted this review to this board not deployed under a directory using these changes.