add mainRef back in

This was previously in the .pageInfo widget, probably not for any good reason, but I'm moving it to the popups div for probably not a good reason.

It's used to get the parent .pane div, and pull its `height` value which is then passed to the `.brewRenderer` div to set the height.

I tried alternate methods that would make this obsolete (CSS methods mostly), but due to some complexity am just ditching those alternatives and popping this sucker into `.popups` for now.

client/homebrew/brewRenderer/brewRenderer.jsx

@@ -205,7 +205,7 @@ const BrewRenderer = (props)=>{
 				: null}
 
 			<ErrorBar errors={props.errors} />
-			<div className='popups'>
+			<div className='popups' ref={mainRef}>
 				<RenderWarnings />
 				<NotificationPopup />
 			</div>