Remove all usages of default_lang

[UltCombo]

The next version of Harmonic intends to remove the post.default_lang template variable. It is time to replace all of its occurrences with something that actually makes sense.

It looks like we were using it only for categories. We could replace it with config.i18n.default !== post.lang in the templates, but perhaps adding a categories_permalink to the Harmonic's config file would be the best way to go.