Validate presence of a translation locale

Instead of throwing an exception when a default or original translation
is not found, build a translation anyway and fail validation if a locale
is not explicitly set afterward.
Create tracklist from a list of songs
Edition#add_label builds a record instead of creating one
Editions and tracks reference parent names by default

Editions and tracks no longer require to have names. They can reference
the parent album or song, respectively, if translations do not exist.
Fix Edition#add_label test
Add artist associations to tracks and songs