fix 404 and 500 image size in small size screen (#11043)

do it by define Semantic UI image class

Signed-off-by: a1012112796 <1012112796@qq.com>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

templates/status/404.tmpl

@@ -1,7 +1,7 @@
 {{template "base/head" .}}
 {{if .IsRepo}}<div class="repository">{{template "repo/header" .}}</div>{{end}}
 <div class="ui container center">
-	<p style="margin-top: 100px"><img src="{{StaticUrlPrefix}}/img/404.png" alt="404"/></p>
+	<p style="margin-top: 100px"><img class="ui centered image" src="{{StaticUrlPrefix}}/img/404.png" alt="404"/></p>
 	<div class="ui divider"></div>
 	<br>
 	<p>{{.i18n.Tr "error404" | Safe}}

templates/status/500.tmpl

@@ -1,6 +1,6 @@
 {{template "base/head" .}}
 <div class="ui container center">
-	<p style="margin-top: 100px"><img src="{{StaticUrlPrefix}}/img/500.png" alt="500"/></p>
+	<p style="margin-top: 100px"><img class="ui centered image" src="{{StaticUrlPrefix}}/img/500.png" alt="500"/></p>
 	<div class="ui divider"></div>
 	<br>
 	{{if .ErrorMsg}}<p>An error has occurred :</p>