Diff box fixes (#15214)

- Fix misaligned "Show Outdated" buttons via flexbox
- Add hover effect to "Show Outdated" buttons
- Remove overreaching margin from selector .diff-file-box and handle
  cases individually.

Fixes: https://github.com/go-gitea/gitea/issues/15097

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/less/_review.less

@@ -35,11 +35,15 @@
   &:not(.top) {
     margin-bottom: .5em;
   }
+}
 
-  .show-outdated,
-  .hide-outdated {
-    display: block;
-    margin-left: auto;
+.show-outdated,
+.hide-outdated {
+  display: block !important;
+  user-select: none !important;
+
+  &:hover {
+    text-decoration: underline;
   }
 }