Extending CurrentUserSerializer functionality

I'd like to be able to surface custom variables via JSON that I can access in a view.  More specifically, I'd like to add methods to `currentUser`.  The way this is done currently is by adding new `attributes` to the `CurrentUserSerializer`.  These `attributes` are then preloaded as JSON within `ApplicationController`.  I was hoping I could simply do the following in a plugin to add new attributes:

```ruby
class CurrentUserSerializer < BasicUserSerializer
  attributes :my_custom_attribute

  def my_custom_attribute
    true  
  end
end
```

But what this actually did was override the entire `CurrentUserSerializer` and break the app.  Sad times.  I then tried this, knowing that you can use this method with models to reopen and extend them:

```ruby
class ::CurrentUserSerializer
  attributes :my_custom_attribute

  def my_custom_attribute
    true  
  end
end
```

This ended up with the same result, i.e. overrode entire `CurrentUserSerializer` and broke the app.  Then I thought maybe what I really needed to do was extend the `attributes` method.  So, I retried both scenarios above (i.e. `class CurrentUserSerializer < BasicUserSerializer` and `class ::CurrentUserSerializer`) with the following code in a plugin:

```ruby
def attributes
  hash = super
  hash['my_custom_attribute'] = true
  hash
end
```

App broke once again... :frowning:

So that really left me with two options:

 1. Completely monkey-patch `CurrentUserSerializer`.  This works, but the downside here is that if Discourse's `CurrentUserSerializer` changes, then my app breaks.  Not a good option, IMO.
 2. Create a new serializer called `MyCurrentUserSerializer` that inherits from `CurrentUserSerializer` so that I can override `MyCurrentUserSerializer` in a plugin and still keep all the goodness of `CurrentUserSerializer` intact.  Here's what that would look like:

###app/serializers/my_current_user_serializer.rb

```ruby
class MyCurrentUserSerializer < CurrentUserSerializer; end
```

###app/controllers/application_controller.rb

```ruby
def preload_current_user_data
  store_preloaded("currentUser", MultiJson.dump(MyCurrentUserSerializer.new(current_user, scope: guardian, root: false)))
  serializer = ActiveModel::ArraySerializer.new(TopicTrackingState.report([current_user.id]), each_serializer: TopicTrackingStateSerializer)
  store_preloaded("topicTrackingStates", MultiJson.dump(serializer))
end
```

###app/controllers/session_controller.rb

```ruby
def current
  if current_user.present?
    render_serialized(current_user, MyCurrentUserSerializer)
  else
    render nothing: true, status: 404
  end
end
```

Then, in my plugin, I can simply do the following and everything works!

```ruby
class MyCurrentUserSerializer < CurrentUserSerializer

  def attributes
    hash = super
    hash['my_custom_attribute'] = true
    hash
  end
end
```