feat(growth): load data on application start

To make this possible, the following changes were made:

* Use `Task` to start the load process
* Create `ets` table before start the async task
  * This is needed to keep the tables alive after the task process exits
* Adjust the application to start the `Growth.Indicators.Load` task

lib/wabanex/application.ex

@@ -16,9 +16,10 @@ defmodule Wabanex.Application do
       # Start the PubSub system
       {Phoenix.PubSub, name: Wabanex.PubSub},
       # Start the Endpoint (http/https)
-      WabanexWeb.Endpoint
+      WabanexWeb.Endpoint,
       # Start a worker by calling: Wabanex.Worker.start_link(arg)
       # {Wabanex.Worker, arg}
+      {Growth.Indicators.Load, []}
     ]
 
     # See https://hexdocs.pm/elixir/Supervisor.html