Display only existing stats

So if we don't have information regarding gender, age or geozone, stats
regarding those topics will not be shown.

Note we're using `spec/models/statisticable_spec.rb` because having the
same file in `spec/models/concerns` caused the tests to be executed
twice.

Also note the implementation behind the `gender?`, `age?` and `geozone?`
methods is a bit primitive. We might need to make it more robust in the
future.

app/views/budgets/stats/show.html.erb

@@ -40,7 +40,7 @@
 
   <div class="row margin">
     <div class="small-12 medium-3 column sidebar">
-      <%= render "shared/stats/links" %>
+      <%= render "shared/stats/links", stats: @stats %>
 
       <p><strong><%= link_to t("stats.advanced"), "#advanced_statistics" %></strong></p>
       <ul class="menu vertical">