Add in page forum post replying

Behat and unit tests provided/updated. Testing instructions: Create a course Create a teacher and student enrolled into the same course As a teacher, create a new forum in the course with default settings Create a discussion in the forum. As as a student, go to the course and forum created. Select the discussion created When you click 'Reply', confirm a simple form shows below the discussion.  Enter some content in the textarea and submit  Confirm the reply just entered appears below the discussion Confirm  the page jumps to the reply you just entered. Click the 'Reply' button on the content in #8 Select the 'Advanced' link and confirm you get redirected to the old reply page.  Repeat the steps with the different displays available. Additional checks: With Display options: "Display replies flat, with newest first" AND "Display replies flat, with oldest first", confirm the newly created reply is shown at the top and bottom of the list of replies respectively, immediately after the first post.