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

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