NEW Add support for ManyManyThrough relations
NEW Add support for ManyManyThrough relations

Previously relationships defiend as many_many came in a special type
of RelationList - however now this can be one of two types of
RelationList depending on the type of definition, with both being
valid many_many relationships.

This had the unfortunate side effect of seeing the OrderableRows
component in (the least) cease functioning correctly. No longer.

This also has the fortunate bonus of allowing a many_many relationship to
be versioned; where previously while each item in the relationship could
be versioned, the relationship itself could not.

silverstripe-australia/silverstripe-gridfieldextensions,symbiote/silverstripe-gridfieldextensions,dhensby/silverstripe-gridfieldextensions,silverstripe-australia/silverstripe-gridfieldextensions,dhensby/silverstripe-gridfieldextensions,symbiote/silverstripe-gridfieldextensions,kinglozzer/silverstripe-gridfieldextensions,kinglozzer/silverstripe-gridfieldextensions