Fix post-commit review request creation on local sites.

The post-commit UI on the new review request page wasn't passing through the
local site prefix. This change fixes that.

Testing done:
Created a local site, and added a repo to it. Loaded /s/site/r/new/ and created
a post-commit review request.

Reviewed at https://reviews.reviewboard.org/r/6143/
Merge branch 'release-2.0.x'