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).

modules/doctor/paths.go

@@ -67,7 +67,7 @@ func checkConfigurationFiles(logger log.Logger, autofix bool) error {
 		return err
 	}
 
-	setting.NewContext()
+	setting.LoadFromExisting()
 
 	configurationFiles := []configurationFile{
 		{"Configuration File Path", setting.CustomConf, false, true, false},