enable reg now that BasicAuth is back in place

Activate ```mezzanine.accounts``` in base settings.

Prior to BasicAuth being enabled, I have enabled the app via local.py. Now that BasicAuth is back in the mix, we should be safe to enable this app by default.

If you have added ```INSTALLED_APPS = base.INSTALLED_APPS + ['mezzanine.accounts']``` to local.py, remove it once this is merged in.