Use single config backup file when config.template is used (#3104)

Copy `config.js` to a single `config.js-old` file whenever a
`config.template` is present, instead of using `Date()` as a suffix.
Creating files with a timestamp suffix means that whenever Magic Mirror
is restarted or recreated a new file is written into the config
directory.

Benefits:
* Single backup file will avoid excessive writing of files
* Saves space and usage on SD cards.
* Makes the folder cleaner and easier to overview, compared to ending up
with something like this as time goes on:

![image](9b66a99c-e760-471e-884c-9daa19689702)

js/app.js

@@ -76,7 +76,7 @@ function App() {
 			// save current config.js
 			try {
 				if (fs.existsSync(configFilename)) {
-					fs.copyFileSync(configFilename, `${configFilename}_${Date.now()}`);
+					fs.copyFileSync(configFilename, `${configFilename}-old`);
 				}
 			} catch (err) {
 				Log.warn(`Could not copy ${configFilename}: ${err.message}`);