FIX: Build correct topic list filter (#11473)

* FIX: 'false' value was treated as a truthy value

For example, latest.json?no_subcategories=false used to have set
no_subcategories to the string value of 'false', which is not false.

* DEV: Remove dead code

* FIX: Redirect to /none under the right conditions

These conditions are:
 - neither /all or /none present
 - only for default filter

* FIX: Build correct topic list filter

/none was never added to the topic list filter

* FIX: Do not show count for subcategories if 'none' category

* FIX: preload_key must contain /none if no_subcategories

spec/requests/list_controller_spec.rb

@@ -88,6 +88,9 @@ RSpec.describe ListController do
   end
 
   describe "categories and X" do
+    let(:category) { Fabricate(:category_with_definition) }
+    let(:sub_category) { Fabricate(:category_with_definition, parent_category: category) }
+
     it "returns top topics" do
       Fabricate(:topic, like_count: 1000, posts_count: 100)
       TopTopic.refresh!
@@ -100,6 +103,12 @@ RSpec.describe ListController do
       data = response.parsed_body
       expect(data["topic_list"]["topics"].length).to eq(2)
     end
+
+    it "returns topics from subcategories when no_subcategories=false" do
+      Fabricate(:topic, category: sub_category)
+      get "/c/#{category.slug}/#{category.id}/l/latest.json?no_subcategories=false"
+      expect(response.parsed_body["topic_list"]["topics"].length).to eq(2)
+    end
   end
 
   describe 'titles for crawler layout' do