Render icons using CSS in user permissions

As mentioned in commit 925f04e3f, icon classes make screen readers
announce strange symbols and aren't properly displayed for people who
have changed their preferred font family.

app/assets/stylesheets/account/permissions_list.scss

@@ -7,17 +7,25 @@
     font-size: $small-font-size;
     margin-bottom: $line-height / 2;
 
-    span {
-      color: $text-medium;
-      font-size: rem-calc(12);
+    &::before {
+      font-size: 0.9em;
+      margin-#{$global-right}: 0.2em;
     }
 
-    .icon-check {
-      color: $check;
+    &.permission-allowed {
+      @include has-fa-icon(check, solid);
+
+      &::before {
+        color: $check;
+      }
     }
 
-    .icon-x {
-      color: $delete;
+    &.permission-denied {
+      @include has-fa-icon(times, solid);
+
+      &::before {
+        color: $delete;
+      }
     }
   }
 }