Merge branch 'task/arrows' into 'develop'

Redo sorting arrows

See merge request ispconfig/ispconfig3!1556

Author: Marius Burkard

interface/web/themes/default/assets/stylesheets/ispconfig.css

@@ -340,34 +340,30 @@ thead.dark th.tiny-col {
   text-overflow: ellipsis; }
 
 thead.dark th[data-column] {
-  cursor: pointer;
+    cursor: pointer;
+    position: relative;
+}
+
+thead.dark th[data-column]:before {
+    content: "⇅";
+    position: absolute;
+    right: 15px;
+    vertical-align: middle;
+    line-height: inherit;
+    color: #aaa;
 }
 
 thead.dark th[data-column][data-ordered] {
-	font-weight: bold;
-	position: relative;
+    background: linear-gradient(to top, #57646d, #3e474e);
 }
 
 thead.dark th[data-column][data-ordered]:before {
-	content: "";
-	display: block;
-	position: absolute;
-	right: 5px;
-	top: 16px;
-	width: 0;
-	height: 0;
-	vertical-align: middle;
-	border-bottom: 5px solid #fff;
-	border-top: 5px solid transparent;
-	border-right: 5px solid transparent;
-	border-left: 6px solid transparent;
-	line-height: inherit;
+    color: #fff;
+    content: "🡫";
 }
 
 thead.dark th[data-column][data-ordered="desc"]:before {
-	top: 21px;
-	border-bottom: 5px solid transparent;
-	border-top: 5px solid #fff;
+    content: "🡩";
 }
 
 thead.dark td input,