Switch to new flexbox spec to improve redraw performance

With the old flexbox, whenever we updated the status bar, other flex
items in the pane (such as the tabs and the *entire* editor) were
getting repainted, which was taking 10x longer than it needed to. This
newer flexbox spec seems to be implemented in a more performant way.

static/editor.css

@@ -1,13 +1,12 @@
 .editor {
-  height: 100%;
   overflow: hidden;
   cursor: default;
   -webkit-user-select: none;
-  -webkit-box-flex: 1;
   position: relative;
   z-index: 0;
   font-family: Inconsolata, Monaco, Courier;
   line-height: 1.3;
+  -webkit-flex: 1;
 }
 
 .editor.mini {
@@ -104,7 +103,6 @@
   right: 0;
   overflow-x: auto;
   overflow-y: hidden;
-  -webkit-box-flex: 1;
 }
 
 .editor.mini .scroll-view {