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/comments_controller_test.rb

@@ -110,7 +110,7 @@ class CommentsControllerTest < ActionDispatch::IntegrationTest
       should "render" do
         @comment = create(:comment, post: @post)
         get comment_path(@comment.id)
-        assert_response :success
+        assert_redirected_to post_path(@comment.post, anchor: "comment_#{@comment.id}")
       end
     end