Fix some UI jumpiness on scroll lock

• Removes body scrollbar in chat layout, the scrolling happens on another element so it's not necessary. on the contrary having it disappear would change UI width in this layout and at right widths (between 800px and 980px) would even scroll the chat when opening and closing images.
• keep nav bar still on scroll lock, currently you can see it shift around as scrollbar disappears
• add really quick fade for media modal images, mostly to hide the image size shifting in a split second sometimes when opening an image

the background image still jumps a bit but I don't know yet how to solve it so pushing for these changes as they are, the chat one especially bothers me a lot currently