Add translatable Powered by Gitea text in footer (#9600)

options/locale/locale_en-US.ini

@@ -10,6 +10,7 @@ link_account = Link Account
 register = Register
 website = Website
 version = Version
+powered_by = Powered by %s
 page = Page
 template = Template
 language = Language

templates/base/footer_content.tmpl

@@ -1,7 +1,7 @@
 <footer>
 	<div class="ui container">
 		<div class="ui left">
-			© Gitea {{if (or .ShowFooterVersion .PageIsAdmin)}}{{.i18n.Tr "version"}}: {{AppVer}}{{end}} {{if ShowFooterTemplateLoadTime}}{{.i18n.Tr "page"}}: <strong>{{LoadTimes .PageStartTime}}</strong> {{.i18n.Tr "template"}}: <strong>{{call .TmplLoadTimes}}</strong>{{end}}
+			{{.i18n.Tr "powered_by" "Gitea"}} {{if (or .ShowFooterVersion .PageIsAdmin)}}{{.i18n.Tr "version"}}: {{AppVer}}{{end}} {{if ShowFooterTemplateLoadTime}}{{.i18n.Tr "page"}}: <strong>{{LoadTimes .PageStartTime}}</strong> {{.i18n.Tr "template"}}: <strong>{{call .TmplLoadTimes}}</strong>{{end}}
 		</div>
 		<div class="ui right links">
 			{{if .ShowFooterBranding}}