Add simple update checker to Gitea (#17212)

* Add simple update checker to Gitea

* update struct and remove comments

* fix lint

* Update custom/conf/app.example.ini

* Update docs/content/doc/advanced/config-cheat-sheet.en-us.md

Co-authored-by: delvh <dev.lh@web.de>

* Update custom/conf/app.example.ini

Co-authored-by: delvh <dev.lh@web.de>

* Update docs/content/doc/advanced/config-cheat-sheet.en-us.md

Co-authored-by: delvh <dev.lh@web.de>

* Update docs/content/doc/advanced/config-cheat-sheet.en-us.md

Co-authored-by: Steven <61625851+justusbunsi@users.noreply.github.com>

* Update docs/content/doc/advanced/config-cheat-sheet.en-us.md

* Update modules/cron/tasks_extended.go

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

* Update custom/conf/app.example.ini

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

* take PR feedback into account and display banner on admin dashboard for alerts

* Add more detailed message

* placate lint

* update per feedback

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Steven <61625851+justusbunsi@users.noreply.github.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/admin/dashboard.tmpl

@@ -3,6 +3,11 @@
 	{{template "admin/navbar" .}}
 	<div class="ui container">
 		{{template "base/alert" .}}
+		{{if .NeedUpdate}}
+			<div class="ui negative message flash-error">
+				<p>"Gitea {{.RemoteVersion | Str2html}} is now available, you are running {{.AppVer | Str2html}}. Check the <a href="https://blog.gitea.io">blog</a> for more details.</p>
+			</div>
+		{{end}}
 		<h4 class="ui top attached header">
 			{{.i18n.Tr "admin.dashboard.statistic"}}
 		</h4>