Allow managers to read investment suggestions

When creating a budget investment with an unverified manager (for
example, a manager who isn't part of the local census), there's a
request to `Budgets::InvestmentsController#suggest`. Since the manager
isn't verified, suggestions can't be obtained.

There are serveral ways to fix this problem:

* Add a `suggest` action to Management::Budgets::InvestmentsController,
doing the same thing the main `suggest` action does.
* Give unverified users permission to access investment suggestions
* Give managers permission to access investment suggestions

I've chosen the last one because I thought it was simple and only
changed existing behaviour for managers, but any other solution would be
as valid. I haven't added the `phase: "accepting"` condition to keep it
simple, since a read-only action like this one in the management portal
isn't gonna create security risks.

spec/features/management/budget_investments_spec.rb

@@ -94,6 +94,29 @@ describe "Budget Investments" do
 
       expect(page).to have_content "User is not verified"
     end
+
+    scenario "Shows suggestions to unverified managers", :js do
+      expect(manager.user.level_two_or_three_verified?).to be false
+
+      create(:budget_investment, budget: budget, title: "More parks")
+      create(:budget_investment, budget: budget, title: "No more parks")
+      create(:budget_investment, budget: budget, title: "Plant trees")
+      login_managed_user(create(:user, :level_two))
+
+      click_link "Create budget investment"
+      within "#budget_#{budget.id}" do
+        click_link "Create budget investment"
+      end
+
+      fill_in "Title", with: "Park"
+      fill_in_ckeditor "Description", with: "Wish I had one"
+
+      within(".js-suggest") do
+        expect(page).to have_content "More parks"
+        expect(page).to have_content "No more parks"
+        expect(page).not_to have_content "Plant trees"
+      end
+    end
   end
 
   context "Searching" do