Fix sticky diff stats container (#12002)

* Fix sticky diff stats container

* Use pure CSS sticky instead of Fomantic's JS

* add border color to arc-green

* add slight padding on sides

* make linter happy

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/less/_repository.less

@@ -1567,6 +1567,16 @@
         background: #ffffff;
         line-height: 30px;
 
+        &.sticky {
+            position: sticky;
+            top: 0;
+            z-index: 800;
+            margin-bottom: 10px;
+            border-bottom: 1px solid #d4d4d5;
+            padding-left: 5px;
+            padding-right: 5px;
+        }
+
         > div:after {
             clear: both;
             content: "";