LivePreview: handle .mw-newarticletext element

LivePreview: handle .mw-newarticletext element

Previously, if LivePreview was used during article creation, the throbber
would be first inserted as the beginning of #mw-content-text (above
.mw-newarticletext), and then overlaid over it on subsequent previews.
Since .mw-newarticletext itself wasn't being faded, this looked ugly.

Now .mw-newarticletext is hidden after first preview and treated like
other elements LivePreview updates. (This mimics the behavior of regular
preview.)

Change-Id: I794d4675721280b041858fd1cb8ca26b32a5ed17