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/base/delete_modal_actions.tmpl

@@ -1,10 +1,10 @@
 <div class="actions">
 	<div class="ui red basic inverted cancel button">
-		<i class="remove icon"></i>
+		{{svg "octicon-trashcan"}}
 		{{.i18n.Tr "modal.no"}}
 	</div>
 	<div class="ui green basic inverted ok button">
-		<i class="checkmark icon"></i>
+		{{svg "octicon-check"}}
 		{{.i18n.Tr "modal.yes"}}
 	</div>
 </div>

templates/base/footer_content.tmpl

@@ -5,10 +5,10 @@
 		</div>
 		<div class="ui right links">
 			{{if .ShowFooterBranding}}
-				<a target="_blank" rel="noopener noreferrer" href="https://github.com/go-gitea/gitea"><i class="fa fa-github-square"></i><span class="sr-only">GitHub</span></a>
+				<a target="_blank" rel="noopener noreferrer" href="https://github.com/go-gitea/gitea">{{svg "octicon-mark-github"}}<span class="sr-only">GitHub</span></a>
 			{{end}}
 			<div class="ui language bottom floating slide up dropdown link item">
-				<i class="world icon"></i>
+				{{svg "octicon-globe"}}
 				<div class="text">{{.LangName}}</div>
 				<div class="menu language-menu">
 					{{range .AllLangs}}