Non-English editions do not get a slug

Because we cannot reliably generate readable slug strings from UTF-8
foreign languages titles, Documents belonging to non-English
are saved without a slug, meaning they fallback to their id for
identification.
Re-organise model code for non-English editions

Rather than stuffing the code directly in the Edition model, try and
keep it local to the appropriate concern.
Don't generate slug without a sluggable_string

if `should_generate_new_friendly_id?` returns true, friendly id will
save a nil-based slug. For example, if a document of the same type
already exists with a nil slug, or one of the format '--%', then
friendly id will save the next sequential slug.