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.

models/activities/statistic.go

@@ -21,7 +21,7 @@ import (
 type Statistic struct {
 	Counter struct {
 		User, Org, PublicKey,
-		Repo, Watch, Star, Action, Access,
+		Repo, Watch, Star, Access,
 		Issue, IssueClosed, IssueOpen,
 		Comment, Oauth, Follow,
 		Mirror, Release, AuthSource, Webhook,
@@ -55,7 +55,6 @@ func GetStatistic() (stats Statistic) {
 	stats.Counter.Repo, _ = repo_model.CountRepositories(db.DefaultContext, repo_model.CountRepositoryOptions{})
 	stats.Counter.Watch, _ = e.Count(new(repo_model.Watch))
 	stats.Counter.Star, _ = e.Count(new(repo_model.Star))
-	stats.Counter.Action, _ = db.EstimateCount(db.DefaultContext, new(Action))
 	stats.Counter.Access, _ = e.Count(new(access_model.Access))
 
 	type IssueCount struct {
@@ -83,7 +82,7 @@ func GetStatistic() (stats Statistic) {
 			Find(&stats.Counter.IssueByRepository)
 	}
 
-	issueCounts := []IssueCount{}
+	var issueCounts []IssueCount
 
 	_ = e.Select("COUNT(*) AS count, is_closed").Table("issue").GroupBy("is_closed").Find(&issueCounts)
 	for _, c := range issueCounts {