Make SVG size argument optional (#12814)

Now defaults to 16 on both frontend and backend.

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

templates/repo/diff/compare.tmpl

@@ -13,7 +13,7 @@
 			{{ end }}
 		</h2>
 		<div class="ui segment choose branch">
-			{{svg "octicon-git-compare" 16}}
+			{{svg "octicon-git-compare"}}
 			<div class="ui floating filter dropdown" data-no-results="{{.i18n.Tr "repo.pulls.no_results"}}">
 				<div class="ui basic small button">
 					<span class="text">{{.i18n.Tr "repo.pulls.compare_base"}}: {{$.BaseName}}:{{$.BaseBranch}}</span>