Extract method to get the stats key in stats

This way we remove a bit of duplication and it'll be easier to change
the `stats_cache` method.

app/models/budget/stats.rb

@@ -192,7 +192,7 @@ class Budget::Stats
 
     stats_cache(*stats_methods)
 
-    def stats_cache(key, &)
-      Rails.cache.fetch("budgets_stats/#{budget.id}/#{phases.join}/#{key}/#{version}", &)
+    def full_cache_key_for(key)
+      "budgets_stats/#{budget.id}/#{phases.join}/#{key}/#{version}"
     end
 end

app/models/concerns/statisticable.rb

@@ -224,4 +224,8 @@ module Statisticable
         I18n.t("stats.age_range", start: start, finish: finish)
       end
     end
+
+    def stats_cache(key, &)
+      Rails.cache.fetch(full_cache_key_for(key), &)
+    end
 end

app/models/poll/stats.rb

@@ -121,7 +121,7 @@ class Poll::Stats
 
     stats_cache(*stats_methods)
 
-    def stats_cache(key, &)
-      Rails.cache.fetch("polls_stats/#{poll.id}/#{key}/#{version}", &)
+    def full_cache_key_for(key)
+      "polls_stats/#{poll.id}/#{key}/#{version}"
     end
 end