Make prepare-dev.py create Djblets media folder in the right place.

prepare-dev.py was attempting to create the Djblets media folder in
reviewboard/htdocs/media/djblets instead of
reviewboard/reviewboard/htdocs/media/djblets.

Reviewed at http://reviews.reviewboard.org/r/2953/

contrib/internal/prepare-dev.py

@@ -73,7 +73,8 @@ def install_media(site):
     print "Using Djblets media from %s" % \
         pkg_resources.resource_filename("djblets", "media")
 
-    site.link_pkg_dir("djblets", "media", os.path.join(media_path, "djblets"))
+    site.link_pkg_dir("djblets", "media",
+                      os.path.join(site.install_dir, media_path, "djblets"))
 
 
 def build_egg_info():