FIX: don't re-enable the locale filter if it wasn't enabled before

This breaks code that tries to get all instances of SiteConfig like:

```
Translatable::disable_locale_filter();
$all = SiteConfig::get();
Translatable::enable_locale_filter();
```

What was happening was that our populateSiteConfigDefaults method was
getting called before the actual data list the users uses above is
created, so it was re-enabling the locale filter before the
augmentDataQuery function was called on the data list created for the
user. Thus, they'd get a locale-augmented query and only get the
SiteConfig for the current locale, and not all as clearly intended.