Include data in chart tags instead of using AJAX

Now that we've moved the logic to generate the events data to a model,
and we've got access to the model in the component rendering the chart,
we can render the data inside the chart instead of doing an extra AJAX
request to get the same data.

Originally, this was problaby done this way so the page wouldn't take
several seconds to load while preparing the data for the chart when
there are thousands of dates being displayed. With an AJAX call, the
page would load as fast as usual, and then the chart would render after
a few seconds. However, we can have an even better performance
improvement in this scenario if we use a Set instead of an Array. The
method `Array#include?`, which we were calling for every date in the
data, is much slower that `Set#merge`. So now both the page and the
chart load as fast as expected.

We could also use something like:

```
def add
  (...)
  shared_keys.push(*collection.keys)
end

def build
  (...)
  shared_keys.uniq.each do |k|
  (...)
end

def shared_keys
  @shared_keys ||= []
end
```

Or other approaches to avoid using `Array#include?`. The performance
would be similar to the one we get when using `Set`. We're using a `Set`
because it makes more obvious that `shared_keys` is supposed to contain
unique elements.

We've had some tests failing in the past due to these AJAX requests
being triggered automatically during the tests and no expectations
checking the requests have finished, so now we're reducing the amount of
flaky tests.

app/controllers/admin/api/stats_controller.rb

@@ -1,9 +0,0 @@
-class Admin::Api::StatsController < Admin::Api::BaseController
-  def show
-    if params[:event].present?
-      render json: Ahoy::Chart.new(params[:event]).data_points
-    else
-      render json: {}, status: :bad_request
-    end
-  end
-end