Replace footer and modal icons with SVG (#13245)

- Replace two font icons with SVG in the footer
- Replace icons in delete modals with SVG
- Horizontally center message text in modals

templates/repo/issue/view_content/sidebar.tmpl

@@ -611,7 +611,7 @@
 
 	<div class="ui basic modal remove-dependency">
 		<div class="ui icon header">
-			<i class="trash icon"></i>
+			{{svg "octicon-trashcan"}}
 			{{.i18n.Tr "repo.issues.dependency.remove_header"}}
 		</div>
 		<div class="content">