Add labels to the search form in the management area

The text for the unfeasible checkbox wasn't correctly defined as a
label, while the fields to search investments and select the heading
weren't intuitive since their purpose wasn't obvious.

spec/components/management/budgets/investments/search_component_spec.rb

@@ -0,0 +1,17 @@
+require "rails_helper"
+
+describe Management::Budgets::Investments::SearchComponent do
+  include Rails.application.routes.url_helpers
+
+  let(:budget) { create(:budget) }
+  let(:url) { management_budget_investments_path(budget) }
+  let(:component) { Management::Budgets::Investments::SearchComponent.new(budget, url: url) }
+
+  it "renders a label for each field" do
+    render_inline component
+
+    expect(page).to have_field "Search investments"
+    expect(page).to have_select "Heading"
+    expect(page).to have_field "Search unfeasible", type: :checkbox
+  end
+end