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/_editor.less

@@ -49,8 +49,8 @@
 .edit-diff > div > .ui.table {
   border-top: none !important;
   border-bottom: none !important;
-  border-left: 1px solid #d4d4d5 !important;
-  border-right: 1px solid #d4d4d5 !important;
+  border-left: 1px solid var(--color-secondary) !important;
+  border-right: 1px solid var(--color-secondary) !important;
 }
 
 #edit_area {