Remove code specific to IE8 and below

Our usage for these browsers in the last month is down to 0.2% of all
users, or 14 individual users, according to Google Analytics.

These users also visit about half the number of pages per sessions,
suggesting that they’re not signed in.

app/assets/stylesheets/components/banner.scss

@@ -30,9 +30,6 @@
 .banner-with-tick {
   padding: $gutter-half ($gutter + $gutter-half);
   background-image: file-url('tick.png');
-  @include ie-lte(8) {
-    background-image: file-url('tick-16px.png');
-  }
   background-size: 19px;
   background-repeat: no-repeat;
   background-position: $gutter-half $gutter-half;
@@ -141,15 +138,7 @@
   }
 
   .greyed-out-step {
-
-    $opacity: 0.6;
-
-    opacity: $opacity;
-
-    @include ie-lte(8) {
-      -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=" + ($opacity * 100) + ")";
-    }
-
+    opacity: 0.6;
   }
 
 }

app/assets/stylesheets/components/checkboxes.scss

@@ -16,10 +16,6 @@
     background-repeat: no-repeat;
     background-size: 39px auto;
     background-position: 0px 4px;
-
-    @include ie-lte(8) {
-      background-image: file-url('folder-black.png');
-    }
   }
 
   // revert full-width button for smaller screens

app/assets/stylesheets/components/letter.scss

@@ -63,36 +63,18 @@ $iso-paper-ratio: 141.42135624%;
     box-shadow: 0 2px 0 0 rgba($border-colour, 0.2);
 
     &-first {
-
       background-image: file-url('envelope-1st-class.svg');
-
-      @include ie-lte(8) {
-        background-image: file-url('envelope-1st-class.png');
-      }
-
     }
 
     &-second {
-
       background-image: file-url('envelope-2nd-class.svg');
-
-      @include ie-lte(8) {
-        background-image: file-url('envelope-2nd-class.png');
-      }
-
     }
 
     .letter-sent &:hover {
-
       background-color: transparent;
       border-color: transparent;
       box-shadow: none;
       background-image: file-url('envelope-fold.svg');
-
-      @include ie-lte(8) {
-        background-image: file-url('envelope-fold.png');
-      }
-
     }
 
   }

app/assets/stylesheets/components/list-entry.scss

@@ -4,10 +4,6 @@
     vertical-align: middle;
     margin-bottom: 15px;
     position: relative;
-
-    @include ie-lte(7) {
-      zoom: 1;
-    }
   }
 
   .list-entry-remove {
@@ -25,11 +21,6 @@
       top: 0;
       left: 100%;
     }
-
-    @include ie-lte(7) {
-      top: auto;
-      left: 110%;
-    }
   }
 
   .form-control {
@@ -37,10 +28,6 @@
     width: 100%;
     margin-bottom: 0;
 
-    @include ie-lte(8) {
-      width: 95%;
-    }
-
     @include media(desktop) {
       @include inline-block;
     }

app/assets/stylesheets/components/message.scss

@@ -46,17 +46,10 @@
       }
 
       &:focus {
-
         // Use box shadow instead of outline to avoid buggy outline
         // rendering in Firefox
         outline: none;
         box-shadow: 0 0 0 3px $yellow;
-
-        @include ie-lte(8) {
-          // Box shadow is not supported in IE8
-          outline: 3px solid $yellow;
-        }
-
       }
 
       .message-name-separator {
@@ -148,18 +141,8 @@
     background-size: auto 20px;
     background-position: 0px 4px;
 
-    @include ie-lte(8) {
-      background-image: file-url('folder-blue-bold.png');
-    }
-
     &:hover {
-
       background-image: file-url('folder-blue-bold-hover.svg');
-
-      @include ie-lte(8) {
-        background-image: file-url('folder-blue-bold-hover.png');
-      }
-
     }
 
   }
@@ -215,14 +198,8 @@
   }
 
   &-folder {
-
     padding: 0 0 0 40px;
     background-image: file-url('folder-black-bold.svg');
-
-    @include ie-lte(8) {
-      background-image: file-url('folder-black-bold.png');
-    }
-
   }
 
   &-folder-truncated {
@@ -252,10 +229,6 @@
       overflow: hidden;
       text-overflow: ellipsis;
 
-      @include ie-lte(8) {
-        background-image: file-url('folder-blue-bold.png');
-      }
-
       &:hover {
         background-image: file-url('folder-blue-bold-hover.svg');
       }

app/assets/stylesheets/components/stick-at-top-when-scrolling.scss

@@ -20,17 +20,11 @@ $sticky-padding: $gutter * 2 / 3;
   }
 
   .back-to-top-link {
-
     position: absolute;
     top: $gutter;
     right: $gutter-half;
     opacity: 0;
     transition: opacity 0.1s ease-in-out;
-
-    @include ie-lte(8) {
-      display: none;
-    }
-
   }
 
 }

app/assets/stylesheets/components/tick-cross.scss

@@ -1,42 +1,24 @@
 %tick-cross {
-
   @include core-19;
   display: inline-block;
   background-size: 19px 19px;
   background-repeat: no-repeat;
   background-position: 0 6px;
   padding: 6px 0 5px 25px;
-
-  @include ie-lte(8) {
-    background-position: 0 3px;
-  }
-
 }
 
 .tick-cross {
 
   &-tick {
-
     @extend %tick-cross;
     background-image: file-url('tick.png');
-
-    @include ie-lte(8) {
-      background-image: file-url('tick-16px.png');
-    }
-
   }
 
   &-cross {
-
     @extend %tick-cross;
     color: $secondary-text-colour;
     background-image: file-url('cross-grey.png');
     box-shadow: inset 20px 0 0 0 rgba(255, 255, 255, 0.6);
-
-    @include ie-lte(8) {
-      background-image: file-url('cross-grey-16px.png');
-    }
-
   }
 
   &-list {