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/unittest/testdb.go

@@ -5,6 +5,7 @@
 package unittest
 
 import (
+	"context"
 	"fmt"
 	"net/url"
 	"os"
@@ -124,7 +125,7 @@ func CreateTestEngine(opts FixturesOptions) error {
 		return err
 	}
 	x.SetMapper(names.GonicMapper{})
-	db.SetEngine(x)
+	db.SetDefaultEngine(context.Background(), x)
 
 	if err = db.SyncAllTables(); err != nil {
 		return err