Misc UI fixes, add secondary color (#13378)

* Misc UI fixes, add secondary color

- Add secondary color, primarily used in arc-green currently
- Convert icons on release page to SVG
- Improve resolved conversation placeholder
- Diff fixes on arc-green
- Misc color tweaks

* fix comment header, adjust arc-green dropzone

* label margin, sidebar margin

* flexbox commits table and add primary button styles

* tooltip styles

* file header fixes

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: zeripath <art27@cantab.net>

web_src/less/_review.less

@@ -27,7 +27,7 @@
 }
 
 .add-comment-left.add-comment-right .ui.attached.header {
-  border: 1px solid #d4d4d5;
+  border: 1px solid var(--color-secondary);
 
   &:not(.top) {
     margin-bottom: .5em;
@@ -89,10 +89,6 @@
     margin: .5em;
   }
 
-  .comment-list {
-    padding-bottom: 5px;
-  }
-
   .footer {
     border-top: 1px solid #f1f1f1;
     padding: 10px 0;