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/_admin.less

@@ -44,7 +44,7 @@
 
   .ui.header,
   .ui.segment {
-    box-shadow: 0 1px 2px 0 rgba(34, 36, 38, .15);
+    box-shadow: 0 1px 2px 0 var(--color-secondary);
   }
 
   &.user {