Prevent accidential selection of line numbers in code view (#1860)

public/css/index.css

@@ -1404,6 +1404,10 @@ footer .ui.language .menu {
   color: #999;
   background: #f5f5f5;
   width: 1%;
+  -moz-user-select: none;
+  -ms-user-select: none;
+  -webkit-user-select: none;
+  user-select: none;
 }
 .repository.file.list #file-content .code-view .lines-num span {
   line-height: 20px;

public/less/_repository.less

@@ -274,6 +274,10 @@
 					color: #999;
 					background: #f5f5f5;
 					width: 1%;
+					-moz-user-select: none;
+					-ms-user-select: none;
+					-webkit-user-select: none;
+					user-select: none;
 
 					span {
 						line-height: 20px;