FIX: if mandatory parent tag is missing, add it

Previous behaviour was to silently remove tags that
belonged to a group with a parent tag that was missing.

The "required parent tag" feature is meant to guide people
to use the correct tags and show scoped results in the tag
input field, and to help create topic lists of related
tags. It isn't meant to be a strict requirement in the
composer that should trigger errors or restrictions.

spec/integration/category_tag_spec.rb

@@ -196,7 +196,7 @@ describe "category tag restrictions" do
   end
 
   context "tag groups with parent tag" do
-    it "filter_allowed_tags returns results based on whether parent tag is present or not" do
+    it "for input field, filter_allowed_tags returns results based on whether parent tag is present or not" do
       tag_group = Fabricate(:tag_group, parent_tag_id: tag1.id)
       tag_group.tags = [tag3, tag4]
       expect(filter_allowed_tags(for_input: true)).to contain_exactly(tag1, tag2)
@@ -204,6 +204,14 @@ describe "category tag restrictions" do
       expect(filter_allowed_tags(for_input: true, selected_tags: [tag1.name, tag3.name])).to contain_exactly(tag2, tag4)
     end
 
+    it "for tagging a topic, filter_allowed_tags allows tags without parent tag" do
+      tag_group = Fabricate(:tag_group, parent_tag_id: tag1.id)
+      tag_group.tags = [tag3, tag4]
+      expect(filter_allowed_tags(for_topic: true)).to contain_exactly(tag1, tag2, tag3, tag4)
+      expect(filter_allowed_tags(for_topic: true, selected_tags: [tag1.name])).to contain_exactly(tag1, tag2, tag3, tag4)
+      expect(filter_allowed_tags(for_topic: true, selected_tags: [tag1.name, tag3.name])).to contain_exactly(tag1, tag2, tag3, tag4)
+    end
+
     context "and category restrictions" do
       let(:car_category)    { Fabricate(:category) }
       let(:other_category)  { Fabricate(:category) }