Use a separate admin page to show global stats, remove actions stat (#25062)

Before, Gitea shows the database table stats on the `admin dashboard`
page.

It has some problems:

* `count(*)` is quite heavy. If tables have many records, this blocks
loading the admin page blocks for a long time
* Some users had even reported issues that they can't visit their admin
page because this page causes blocking or `50x error (reverse proxy
timeout)`
* The `actions` stat is not useful. The table is simply too large. Does
it really matter if it contains 1,000,000 rows or 9,999,999 rows?
* The translation `admin.dashboard.statistic_info` is difficult to
maintain.

So, this PR uses a separate page to show the stats and removes the
`actions` stat.


![image](https://github.com/go-gitea/gitea/assets/2114189/babf7c61-b93b-4a62-bfaa-22983636427e)

## ⚠️ BREAKING

The `actions` Prometheus metrics collector has been removed for the
reasons mentioned beforehand.
Please do not rely on its output anymore.

routers/web/web.go

@@ -538,8 +538,8 @@ func registerRoutes(m *web.Route) {
 
 	// ***** START: Admin *****
 	m.Group("/admin", func() {
-		m.Get("", adminReq, admin.Dashboard)
-		m.Post("", adminReq, web.Bind(forms.AdminDashboardForm{}), admin.DashboardPost)
+		m.Get("", admin.Dashboard)
+		m.Post("", web.Bind(forms.AdminDashboardForm{}), admin.DashboardPost)
 
 		m.Group("/config", func() {
 			m.Get("", admin.Config)
@@ -548,6 +548,7 @@ func registerRoutes(m *web.Route) {
 		})
 
 		m.Group("/monitor", func() {
+			m.Get("/stats", admin.MonitorStats)
 			m.Get("/cron", admin.CronTasks)
 			m.Get("/stacktrace", admin.Stacktrace)
 			m.Post("/stacktrace/cancel/{pid}", admin.StacktraceCancel)