Show valuator group investments to their valuators

When accessing the valuation area, we were only displaying the
investments directly assigned to the current valuator, but we weren't
displaying the investments assigned to that valuator's group.

Using the `assigned_investments_ids` method, which takes the valuator
group into account, solves the issue.

We've also found an issue on our development machines: since we don't
have a unique index per `investment_id` and `valuator_id` in the
`budget_valuator_assignments` table, we've found duplicate records on
this table. When that happened, we were displaying the same investment
several times.

Since now we no longer join this table in the query returning the
investment, this issue is also solved, and we're adding a test for it.
We can now remove the call to the `distinct` method when calculating the
number of investments per heading.

spec/models/budget/investment_spec.rb

@@ -414,6 +414,55 @@ describe Budget::Investment do
     end
   end
 
+  describe ".visible_to_valuator" do
+    let(:valuator) { create(:valuator) }
+
+    it "returns investments assigned to the valuator" do
+      investment = create(:budget_investment, :visible_to_valuators, valuators: [valuator])
+
+      expect(Budget::Investment.visible_to_valuator(valuator)).to eq [investment]
+    end
+
+    it "does not return investments assigned to other valuators" do
+      create(:budget_investment, :visible_to_valuators, valuators: [create(:valuator)])
+
+      expect(Budget::Investment.visible_to_valuator(valuator)).to be_empty
+    end
+
+    it "does not return duplicate investments when they're assigned more than once" do
+      investment = create(:budget_investment, :visible_to_valuators)
+      2.times { Budget::ValuatorAssignment.create!(valuator: valuator, investment: investment) }
+
+      expect(Budget::Investment.visible_to_valuator(valuator)).to eq [investment]
+    end
+
+    it "does not return duplicate investments when assigned to both a valuator and their group" do
+      valuator_group = create(:valuator_group, valuators: [valuator])
+      investment = create(:budget_investment, :visible_to_valuators, valuators: [valuator],
+                                                                     valuator_groups: [valuator_group])
+
+      expect(Budget::Investment.visible_to_valuator(valuator)).to eq [investment]
+    end
+
+    it "returns investments assigned to the valuator's group" do
+      valuator_group = create(:valuator_group, valuators: [valuator])
+      investment = create(:budget_investment, :visible_to_valuators, valuator_groups: [valuator_group])
+
+      expect(Budget::Investment.visible_to_valuator(valuator)).to eq [investment]
+    end
+
+    it "does not return investments assigned to other valuator groups" do
+      valuator_group = create(:valuator_group, valuators: [create(:valuator)])
+      create(:budget_investment, :visible_to_valuators, valuator_groups: [valuator_group])
+
+      expect(Budget::Investment.visible_to_valuator(valuator)).to be_empty
+    end
+
+    it "returns an empty relation when valuator is nil" do
+      expect(Budget::Investment.visible_to_valuator(nil)).to be_empty
+    end
+  end
+
   describe ".scoped_filter" do
     let(:budget)     { create(:budget, :balloting, slug: "budget_slug") }
     let(:investment) { create(:budget_investment, budget: budget) }