forum posts, comments: make timestamps into permalinks.

Make the timestamp beneath the username on forum posts into a permalink
that links to the post in full context of the thread. For comments, make
the timestamp link to the comment in full context of the post.

* Make the timestamp in forum posts link to /forum_posts/123.
* Make the timestamp in comments link to /posts/456#comment_123.
* Make /forum_posts/123 redirect to /forum_topics/456#forum_post_123.
* Make /comments/123 redirect to /posts/456#comment_123.
* Remove the "ID: ###" and "Permalink" fields from forum posts.

test/functional/forum_posts_controller_test.rb

@@ -110,6 +110,11 @@ class ForumPostsControllerTest < ActionDispatch::IntegrationTest
 
         assert_response 403
       end
+
+      should "redirect to the forum topic" do
+        get forum_post_path(@forum_post)
+        assert_redirected_to forum_topic_path(@forum_post.topic, anchor: "forum_post_#{@forum_post.id}")
+      end
     end
 
     context "edit action" do