Comment box tweaks and SVG dropdown triangles (#13376)

* Comment box tweaks and SVG dropdown triangles

- Change all dropdown triangles to SVG
- Bring inline review comment box closer to regular comment boxes
- Enhance arc-green checkbox contrast
- Minor reaction tweaks
- Flexbox the diff file header

* remove a border

* fix type marker in arc-green

* add small code padding

* fix position regression and remove useless rules

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/less/_form.less

@@ -158,19 +158,8 @@
     #create-page-form;
 
     form {
-      .dropdown {
-        .dropdown.icon {
-          margin-top: -7px !important;
-          padding-bottom: 5px;
-        }
-
-        .text {
-          margin-right: 0 !important;
-
-          i {
-            margin-right: 0 !important;
-          }
-        }
+      .dropdown .text {
+        margin-right: 0 !important;
       }
 
       .header {