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).

cmd/convert.go

@@ -35,7 +35,6 @@ func runConvert(ctx *cli.Context) error {
 	log.Info("Custom path: %s", setting.CustomPath)
 	log.Info("Log path: %s", setting.LogRootPath)
 	log.Info("Configuration file: %s", setting.CustomConf)
-	setting.InitDBConfig()
 
 	if !setting.Database.UseMySQL {
 		fmt.Println("This command can only be used with a MySQL database")