This change aims to improve the initialization of Review Board (through

the initialize() method), to give callers more control over that

process, and to reduce the work needed by callers.
initialize() now has a few new arguments for controlling the

initialization process. Extensions can now be turned off for callers

that don't need them, as can some custom template support ({% url %}

tag on Django 1.6 and some template serial generation for caching).

Logging can also be turned off, in case the caller has set things up

already.
Callers no longer need to set the DJANGO_SETTINGS_MODULE environment

variable first. If not already set, initialize() will set this to

reviewboard.settings automatically.
We also already had some code that attempted to initialize Django (using

django.setup()), but it was doing this a bit later than we wanted. We

now try to do this earlier, before we import anything from Django. We

also only do it now if we've detected that Django isn't yet set up.
rbssh has been updated to take advantage of these changes as well,

removing all extension loading, template serial generation, and unwanted

logging setup (which is being overridden anyway) to minimize startup

time.

Unit tests pass on Python 2.7/Django 1.6 and Python 3.7/Django 1.11.
Manually tested rbssh with and without debug logging.