Improve install code to avoid low-level mistakes. (#17779)

* Improve install code to avoid low-level mistakes.

If a user tries to do a re-install in a Gitea database, they gets a warning and double check.
When Gitea runs, it never create empty app.ini automatically.

Also some small (related) refactoring:

* Refactor db.InitEngine related logic make it more clean (especially for the install code)
* Move some i18n strings out from setting.go to make the setting.go can be easily maintained.
* Show errors in CLI code if an incorrect app.ini is used.
* APP_DATA_PATH is created when installing, and checked when starting (no empty directory is created any more).

contrib/environment-to-ini/environment-to-ini.go

@@ -156,6 +156,7 @@ func runEnvironmentToIni(c *cli.Context) error {
 		destination = setting.CustomConf
 	}
 	if destination != setting.CustomConf || changed {
+		log.Info("Settings saved to: %q", destination)
 		err = cfg.SaveTo(destination)
 		if err != nil {
 			return err

contrib/pr/checkout.go

@@ -49,7 +49,7 @@ func runPR() {
 		log.Fatal(err)
 	}
 	setting.SetCustomPathAndConf("", "", "")
-	setting.NewContext()
+	setting.LoadAllowEmpty()
 
 	setting.RepoRootPath, err = os.MkdirTemp(os.TempDir(), "repos")
 	if err != nil {