Replace code fold icons with octicons (#12222)

- replace font-awesome icons with octicons
- clean up js and css surrounding the code expansion and file folding
- fix hover color on arc-green
- tweak diff line number colors

Co-authored-by: zeripath <art27@cantab.net>

web_src/less/_review.less

@@ -133,27 +133,24 @@
     color: rgba(0, 0, 0, .87);
 }
 
-.ui.fold-code {
-    margin-right: 1em;
-    padding-left: 5px;
-    cursor: pointer;
-    width: 22px;
-    font-size: 12px;
+a.fold-file {
+    margin-right: 10px;
+    color: inherit;
 }
 
-.ui.fold-code:hover {
-    color: #428bca;
+a.blob-excerpt {
+    color: #575a68;
+    height: 28px;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    width: 100%;
+    background: #daecfe;
 }
 
-.ui.blob-excerpt {
-    display: block;
-    line-height: 20px;
-    font-size: 16px;
-    cursor: pointer;
-}
-
-.ui.blob-excerpt:hover {
-    color: #428bca;
+a.blob-excerpt:hover {
+    background: #428bca;
+    color: #fff;
 }
 
 .btn-review > .dropdown.icon {