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/serv.go

@@ -58,7 +58,7 @@ func setup(logPath string, debug bool) {
 	} else {
 		_ = log.NewLogger(1000, "console", "console", `{"level":"fatal","stacktracelevel":"NONE","stderr":true}`)
 	}
-	setting.NewContext()
+	setting.LoadFromExisting()
 	if debug {
 		setting.RunMode = "dev"
 	}