adds validation for custom dates in advanced search

avoids error 500 on invalid dates

spec/features/proposals_spec.rb

@@ -980,6 +980,28 @@ feature 'Proposals' do
           end
         end
 
+        scenario "Search by custom invalid date range", :js do
+          proposal1 = create(:proposal, created_at: 2.days.ago)
+          proposal2 = create(:proposal, created_at: 3.days.ago)
+          proposal3 = create(:proposal, created_at: 9.days.ago)
+
+          visit proposals_path
+
+          click_link "Advanced search"
+          select "Customized", from: "js-advanced-search-date-min"
+          fill_in "advanced_search_date_min", with: 4000.years.ago
+          fill_in "advanced_search_date_max", with: "wrong date"
+          click_button "Filter"
+
+          expect(page).to have_content("There are 3 citizen proposals")
+
+          within("#proposals") do
+            expect(page).to have_content(proposal1.title)
+            expect(page).to have_content(proposal2.title)
+            expect(page).to have_content(proposal3.title)
+          end
+        end
+
         scenario "Search by multiple filters", :js do
           ana  = create :user, official_level: 1
           john = create :user, official_level: 1