Disabled highcontrast image effect by default

Signed-off-by: Blagovest Petrov <blagovest@petrovs.info>

static/css/bootstrap.css

@@ -645,7 +645,7 @@ a:focus {
   clear: both;
 }
 
-img {
+img-highcontrast {
   -webkit-filter: contrast(800%);
      -moz-filter: contrast(1000%);
       -ms-filter: contrast(800%);