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/labels/edit_delete_label.tmpl

@@ -1,6 +1,6 @@
 <div class="ui small basic delete modal">
 	<div class="ui icon header">
-		<i class="trash icon"></i>
+		{{svg "octicon-trashcan"}}
 		{{.i18n.Tr "repo.issues.label_deletion"}}
 	</div>
 	<div class="content">