Add-comment form should respect comment ordering

For adding a new comment, the Comment button and its corresponding form should be displayed according to the ordering (implemented in JRASERVER-3454), i.e:

• after the comments, when ordering is oldest to newest
• before the comments, when ordering is newest to oldest

Currently, the form is always displayed after, so it is very confusing when ordering is newest to oldest: you see the oldest comment and have the impression you reply to that. Yet, most of the times, a comment is related to the newest (latest) comment.

A good example for this behavior is the Facebook comments section.