add `has_and_belongs_to_many`

[catmando]

This should work:

  def self.has_and_belongs_to_many(other, opts = {})
    has_many other, opts.merge(through: [other, table_name].sort.join('_'))
  end if RUBY_ENGINE == 'opal'

[catmando]

This will be added however with a shortfall that you cannot delete items in the habtm association, but must use destroy instead.