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>

templates/repo/diff/comments.tmpl

@@ -27,7 +27,7 @@
 					</span>
 				{{else}}
 					<span class="text grey">
-						<a class="mr-2" {{if gt .Poster.ID 0}}href="{{.Poster.HomeLink}}"{{end}}>
+						<a {{if gt .Poster.ID 0}}href="{{.Poster.HomeLink}}"{{end}}>
 							{{.Poster.GetDisplayName}}
 						</a>
 						{{$.root.i18n.Tr "repo.issues.commented_at" .HashTag $createdStr | Safe}}