FIX: Use a default hashtag icon color for user with no permission (#21825)

One user can create a post or chat message with a hashtag they
have permission to use, but then when other users look at that
post they will see an empty space next to the hashtag because they
do not have the permission to load the colors in CSS classes for
the related category.

This fixes the issue by adding a default color with a special
CSS class if the user doesn't have permission to see the linked
channel/category on the hashtag.

spec/system/hashtag_autocomplete_spec.rb

@@ -93,4 +93,21 @@ describe "Using #hashtag autocompletion to search for and lookup categories and
     <a class=\"hashtag-cooked\" href=\"#{tag.url}\" data-type=\"tag\" data-slug=\"cooltag\" data-id=\"#{tag.id}\"><svg class=\"fa d-icon d-icon-tag svg-icon hashtag-color--tag-#{tag.id} svg-string\" xmlns=\"http://www.w3.org/2000/svg\"><use href=\"#tag\"></use></svg><span>cooltag</span></a>
     HTML
   end
+
+  context "when a user cannot access the category for a hashtag cooked in another post" do
+    fab!(:admin) { Fabricate(:admin) }
+    fab!(:manager_group) { Fabricate(:group, name: "Managers") }
+    fab!(:private_category) do
+      Fabricate(:private_category, name: "Management", slug: "management", group: manager_group)
+    end
+    fab!(:admin_group_user) { Fabricate(:group_user, user: admin, group: manager_group) }
+    fab!(:post_with_private_category) do
+      Fabricate(:post, topic: topic, raw: "this is a secret #management category", user: admin)
+    end
+
+    it "shows a default color and css class for the category icon square" do
+      topic_page.visit_topic(topic, post_number: post_with_private_category.post_number)
+      expect(page).to have_css(".hashtag-cooked .hashtag-missing")
+    end
+  end
 end

spec/system/page_objects/pages/topic.rb

@@ -8,8 +8,10 @@ module PageObjects
         @fast_edit_component = PageObjects::Components::FastEditor.new
       end
 
-      def visit_topic(topic)
-        page.visit "/t/#{topic.id}"
+      def visit_topic(topic, post_number: nil)
+        url = "/t/#{topic.id}"
+        url += "/#{post_number}" if post_number
+        page.visit url
         self
       end