Center-aligned login topbar (#1880)

* Center-aligned login topbar

* Center-align OpenID login

templates/user/auth/signin_inner.tmpl

@@ -1,7 +1,7 @@
 		{{if or (not .LinkAccountMode) (and .LinkAccountMode .LinkAccountModeSignIn)}}
 		{{template "base/alert" .}}
 		{{end}}
-		<h4 class="ui top attached header">
+		<h4 class="ui top attached header center">
 			{{.i18n.Tr "auth.login_userpass"}}
 		</h4>
 		<div class="ui attached segment">

templates/user/auth/signin_openid.tmpl

@@ -3,7 +3,7 @@
 	{{template "user/auth/signin_navbar" .}}
 	<div class="ui container">
 		{{template "base/alert" .}}
-		<h4 class="ui top attached header">
+		<h4 class="ui top attached header center">
 			<i class="fa fa-openid"></i>
 			OpenID
 		</h4>