Allow for default_locale to be nil

[jtomaszewski]

Feature request:

1. If default_locale is false, (or we didnt found a default_locale in `available_locales), then:
   • if object has already some translations, just set as default the first one found
   • if object has no translations (f.e. when its a new_record?), it should show only available_locales list (we need to explicitly add first translation, to make the translatable_inputs) visible

Changes would be need to be done propably in https://github.com/fabn/activeadmin-globalize/blob/develop/lib/active_admin/globalize/form_builder_extension.rb .

I might to do a PR if interested.

[jtomaszewski]

Might be dependant on https://github.com/globalize/globalize/pull/328 , reading through it now.

[jtomaszewski]

It's working good, but when trying to save the object, globalize raises validation error for the translation object of default locale (which shouldnt be created at all). More info in globalize/globalize#328 - we'll need to wait for the fix over there.