Associations: DRY up the code which is generating conditions, and make it all use arel rather than SQL strings

Set the create scope to an empty hash in ThroughAssociationScope. For reasoning please see the inline code comments.

Use conditionals and implicit returns rather than explicit returns and postfix ifs (it's easier to read)

Remove has_cached_counter? and cached_counter_attribute_name from HasManyThroughAssociation, as the exact same methods are inherited from HasManyAssociation

Use the through association proxy for operations on the through record, so that those operations are automatically scoped and therefore construct_join_attributes does not need to use construct_owner_attributes.

see: https://github.com/blog/738-sidejack-prevention-phase-2-ssl-everywhere

[#6223 state:committed]
Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>