URLs that use prefer_cms should merge active_locales data

[alexgibson]

Description

When viewing a page published using Wagtail, the languages listed in the footer's language picker map only to the published languages that Wagtail knows about in the CMS. If the page uses the prefer_cms decorator, then the fallback Django / Fluent languages are not listed in the picker. The same is true in reverse. When viewing a Django / Fluent translated page, the language picker will not include languages that are published in Wagtail.

We should combine this data when rendering a template, so that the language picker lists all available translations for a given URL, regardless to where the source data is published from.