Change HABTM to has_many :through

HABTM is basically bridge to has_many :through now.

Quoting from ActiveRecord changelog (https://github.com/rails/rails/blob/master/activerecord/CHANGELOG.md):

  has_and_belongs_to_many is now transparently implemented in terms of has_many :through.
  Behavior should remain the same, if not, it is a bug.