Fix rb-site (and prepare-dev.py) on Python 3 and Django 1.11.
Fix rb-site (and prepare-dev.py) on Python 3 and Django 1.11.

This addresses some string type consistency issues on Python 3 and
Django environment setup issues on Django 1.11, bringing us working
support for Django 1.11 on Python 3.

We also had to disable Django's built-in warnings checks during rb-site,
in order to not scare off users who think they have something that needs
to be fixed.

Testing Done:
Installed and upgraded a site using Python 3 and Django 1.11.

Reviewed at https://reviews.reviewboard.org/r/10681/

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