Select only distinct voters/balloters in stats

It is way more efficient because we're caching the result of that
method, and this way we only store each voter once in the cache. We were
storing many voters several times and then we were filtering them with
`uniq`.

app/models/budget/stats.rb

@@ -18,7 +18,7 @@ class Budget::Stats
   end
 
   def total_participants_support_phase
-    voters.uniq.count
+    voters.count
   end
 
   def total_participants_vote_phase
@@ -71,11 +71,11 @@ class Budget::Stats
     end
 
     def voters
-      supports(budget).pluck(:voter_id)
+      supports(budget).distinct.pluck(:voter_id)
     end
 
     def balloters
-      budget.ballots.where("ballot_lines_count > ?", 0).pluck(:user_id).compact
+      budget.ballots.where("ballot_lines_count > ?", 0).distinct.pluck(:user_id).compact
     end
 
     def poll_ballot_voters
@@ -84,21 +84,23 @@ class Budget::Stats
 
     def balloters_by_heading(heading_id)
       stats_cache("balloters_by_heading_#{heading_id}") do
-        budget.ballots.joins(:lines).where(budget_ballot_lines: {heading_id: heading_id}).pluck(:user_id)
+        budget.ballots.joins(:lines)
+                      .where(budget_ballot_lines: { heading_id: heading_id} )
+                      .distinct.pluck(:user_id)
       end
     end
 
     def voters_by_heading(heading)
       stats_cache("voters_by_heading_#{heading.id}") do
-        supports(heading).pluck(:voter_id)
+        supports(heading).distinct.pluck(:voter_id)
       end
     end
 
     def calculate_heading_totals(heading)
       {
         total_investments_count: heading.investments.count,
-        total_participants_support_phase: voters_by_heading(heading).uniq.count,
-        total_participants_vote_phase: balloters_by_heading(heading.id).uniq.count,
+        total_participants_support_phase: voters_by_heading(heading).count,
+        total_participants_vote_phase: balloters_by_heading(heading.id).count,
         total_participants_all_phase: voters_and_balloters_by_heading(heading)
       }
     end