Fix flex CSS

We used flex CSS features from both old and new specification.
Furthermore, we weren't using moz and ms prefixes for a new model, which
may have also made it worse in IE and Firefox.

This commit changes CSS to use new flexbox implementation with all the
prefixes (which should not be needed for current versions, but it won't
hurt to keep them for a while).

assets/styles/layout.sass

@@ -4,10 +4,14 @@ $left-width: 250px
 
 @mixin display-flex
   display: -webkit-flex
+  display: -moz-flex
+  display: -ms-flexbox
   display: flex
 
 @mixin flex($grow, $shrink, $size)
+  -ms-flex:     $grow $shrink $size
   -webkit-flex: $grow $shrink $size
+  -moz-flex:    $grow $shrink $size
   flex:         $grow $shrink $size
 
 html, body
@@ -18,9 +22,7 @@ html, body
   width: 100%
   min-height: 100%
   margin-top: 40px
-  display: -webkit-box
   @include display-flex
-  display: -moz-box
 
 #top
   position: absolute
@@ -34,14 +36,9 @@ html, body
 #left, #right
   position: relative
   min-height: 100%
-  -moz-box-flex: 0
-  -webkit-box-flex: 0
+  @include flex(0, 0, 25em)
 
 #left
-  @include flex(1, 1, 30%)
-  min-width: 320px
-  max-width: 320px
-  width: -webkit-calc(100% - 1000px)
   padding: 0 0 110px 0
   background-color: #fbfbfa
   border-right: 1px solid $color-border-normal
@@ -51,9 +48,10 @@ html, body
     max-width: 300px
 
 #main
-  @include flex(1, 1, 70%)
-  -moz-box-flex: 4
-  -webkit-box-flex: 4
+  -webkit-flex: 1
+  -moz-flex:    1
+  -ms-flex:     1
+  flex:         1
   position: relative
   min-width: 539px
   padding: 20px 40px 80px 30px