Fix issues with prepare-dev.py and rb-site.

There were a couple issues in running prepare-dev.py with django 1.6. The first
one was that rb-site was still using execute_manager, which has been deprecated
roughly forever. That has finally been removed, so we need to switch to
execute_from_command_line. Second, we weren't getting a useful error message,
because prepare-dev was invoking commands in rb-site without the 'ui' variable
being set to anything.

Testing done: Ran prepare-dev.py successfully.

Reviewed at https://reviews.reviewboard.org/r/5409/