Include `href` or `to_param` when constructing URLs client side

One nice feature that Rails provides is easy changes to URL generation
(assuming the param starts with the ID). All you have to do is

```ruby
def to_param
  [id, name].to_param
end
```

and everything "Just Works". This is lost, however, if you're manually
constructing URLs on the client side. This is avoided, if as a general
rule of thumb, we let the server decide how to construct the URLs, and
pass that information along.
@seanpdoyle @joshuaclayton ActiveModel::Serializers [doesn't define `to_param` at all](https://github.com/rails-api/active_model_serializers/search?utf8=%E2%9C%93&q=to_param), but ActiveSupport [defines it on every object](https://github.com/rails/rails/blob/08754f12e65a9ec79633a605e986d0f1ffa4b251/activesupport/lib/active_support/core_ext/object/to_param.rb#L3). `ActiveModel::Serializer.attributes` will only define attribute methods which are [aren't already defined](https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model/serializer.rb#L26), so it ends up using the default `to_param`, which is the same as `to_s`. Delegating like @sgrif is above seems like the easiest way to fix this.