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).

models/db/install/db.go

@@ -0,0 +1,65 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package install
+
+import (
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/modules/setting"
+
+	"xorm.io/xorm"
+)
+
+func getXORMEngine() *xorm.Engine {
+	return db.DefaultContext.(*db.Context).Engine().(*xorm.Engine)
+}
+
+// CheckDatabaseConnection checks the database connection
+func CheckDatabaseConnection() error {
+	e := db.GetEngine(db.DefaultContext)
+	_, err := e.Exec("SELECT 1")
+	return err
+}
+
+// GetMigrationVersion gets the database migration version
+func GetMigrationVersion() (int64, error) {
+	var installedDbVersion int64
+	x := getXORMEngine()
+	exist, err := x.IsTableExist("version")
+	if err != nil {
+		return 0, err
+	}
+	if !exist {
+		return 0, nil
+	}
+	_, err = x.Table("version").Cols("version").Get(&installedDbVersion)
+	if err != nil {
+		return 0, err
+	}
+	return installedDbVersion, nil
+}
+
+// HasPostInstallationUsers checks whether there are users after installation
+func HasPostInstallationUsers() (bool, error) {
+	x := getXORMEngine()
+	exist, err := x.IsTableExist("user")
+	if err != nil {
+		return false, err
+	}
+	if !exist {
+		return false, nil
+	}
+
+	// if there are 2 or more users in database, we consider there are users created after installation
+	threshold := 2
+	if !setting.IsProd {
+		// to debug easily, with non-prod RUN_MODE, we only check the count to 1
+		threshold = 1
+	}
+	res, err := x.Table("user").Cols("id").Limit(threshold).Query()
+	if err != nil {
+		return false, err
+	}
+	return len(res) >= threshold, nil
+}