New Comment button

The comments section at the bottom of pages used to have the comment editor visible. There's now a Comment button to click when wanting to add a comment. This was done for 3 reasons:

• For consistency with other actions where you have to click a button to do them (annotate, edit, create, etc).
• It makes the UI a bit less cluttered and avoids focusing the attention on comments which was very visible, especially since we moved to using the WYSIWYG editor by default in comments.
• It also improves the loading performance of XWiki pages since the editor doesn't need to be loaded when viewing pages