templates: update year in footer (#4966)

Happy new year!

templates/base/footer.tmpl

@@ -7,7 +7,7 @@
 	<footer>
 		<div class="ui container">
 			<div class="ui left">
-				© 2017 Gogs {{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}}
+				© 2018 Gogs {{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}}