Media query added

Added media-query for 900px wide maincontent section. Currently, it's maxed at 500px, which squishes some of the tab menus and tables often exceed the 500px and look wonky. This should make the width of the pages 900px when the screen size is 1024px or wider.

To test: open up the admin webUI on a device with a resolution of 1024px or wider. Check out how it looks on a bunch of different pages. Then de-maximize the browser window and narrow it until it goes back to a 500px width.