#32 Add some for artist name variation for release extra artists and tra...

Added support for anv in release extra artists and track extra artists.
Also now initialize anv and join_relation to None so that we don't need separate INSERT statements to check if these fields have values, if don't have value will just insert NULL, i.e have no effect.

(Note currently impossible to define a primary key on releases_extrartists because some releases have multiple ocurences of the same artist only differentiated by their anv, but because anv is usually null it cannot be used as part of the primary key)