Add PNG versions of the folder icons for IE8 users

IE8 doesn’t support SVG images as CSS backgrounds. We still have users
on IE8, as I saw yesterday.

This commit adds fallback PNG images for these users. The images are
rendered at 1x (because no-one is using IE8 on a retina screen) and
have been run through `pngcrush -brute` for the smallest possible file
size.

app/assets/stylesheets/components/message.scss

@@ -87,12 +87,18 @@
   &-folder {
 
     a:first-child {
+
       display: inline-block;
       text-indent: 40px;
       background-image: file-url('folder-blue-bold.svg');
       background-repeat: no-repeat;
       background-size: auto 20px;
       background-position: 0px 2px;
+
+      @include ie-lte(8) {
+        background-image: file-url('folder-blue-bold.png');
+      }
+
     }
 
   }
@@ -134,10 +140,16 @@
     vertical-align: top;
 
     &:first-child {
+
       background-image: file-url('folder-blue.svg');
       max-width: 33%;
       overflow: hidden;
       text-overflow: ellipsis;
+
+      @include ie-lte(8) {
+        background-image: file-url('folder-blue.png');
+      }
+
     }
 
   }
@@ -147,8 +159,14 @@
     padding-left: 0;
 
     &:first-child {
+
       background-image: file-url('folder-black.svg');
       padding-left: $gutter * 2;
+
+      @include ie-lte(8) {
+        background-image: file-url('folder-black.png');
+      }
+
     }
 
   }