CSS: Add fadein animation to mixins

Was in login.less before. Will be removed.

public/css/icinga/mixins.less

@@ -70,3 +70,49 @@
 .visible {
   visibility: visible;
 }
+
+// Fadein animation
+
+/* Chrome, WebKit */
+@-webkit-keyframes fadein {
+  from { opacity: 0; }
+  to   { opacity: 1; }
+}
+
+/* FF < 16 */
+@-moz-keyframes fadein {
+  from { opacity: 0; }
+  to   { opacity: 1; }
+}
+
+/* IE */
+@-ms-keyframes fadein {
+  from { opacity: 0; }
+  to   { opacity: 1; }
+}
+
+/* Opera < 12.1 */
+@-o-keyframes fadein {
+  from { opacity: 0; }
+  to   { opacity: 1; }
+}
+
+@keyframes fadein {
+  from { opacity: 0; }
+  to   { opacity: 1; }
+}
+
+.fadein() {
+  opacity: 0;
+
+  -webkit-animation: fadein 2s ease-in; /* Chrome, WebKit */
+     -moz-animation: fadein 2s ease-in; /* FF < 16 */
+       -o-animation: fadein 2s ease-in; /* Opera < 12.1 */
+          animation: fadein 2s ease-in;
+
+  // Make sure that after animation is done we remain at the last keyframe value (opacity: 1)
+  -webkit-animation-fill-mode: forwards;
+     -moz-animation-fill-mode: forwards;
+       -o-animation-fill-mode: forwards;
+          animation-fill-mode: forwards;
+}