Move budget stats permission tests to the model

They're easier to test and to read in the model.

spec/features/budgets/stats_spec.rb

@@ -7,22 +7,6 @@ feature "Stats" do
   let(:heading) { create(:budget_heading, group: group, price: 1000) }
 
   describe "Show" do
-
-    it "is not accessible if supports phase is not finished" do
-      budget.update(phase: "selecting")
-
-      visit budget_stats_path(budget.id)
-      expect(page).to have_content "You do not have permission to carry out the action "\
-                                   "'read_stats' on budget."
-    end
-
-    it "is accessible if supports phase is finished" do
-      budget.update(phase: "valuating")
-
-      visit budget_stats_path(budget.id)
-      expect(page).to have_content "Stats"
-    end
-
   end
 
 end

spec/models/abilities/everyone_spec.rb

@@ -73,4 +73,18 @@ describe Abilities::Everyone do
       it { should_not be_able_to(:stats, poll) }
     end
   end
+
+  context "when accessing budget stats" do
+    context "supports phase is not finished" do
+      let(:budget) { create(:budget, phase: "selecting") }
+
+      it { should_not be_able_to(:read_stats, budget) }
+    end
+
+    context "supports phase is finished" do
+      let(:budget) { create(:budget, phase: "valuating") }
+
+      it { should be_able_to(:read_stats, budget) }
+    end
+  end
 end