Reworked Backend

Here is my Pull-Request for my Backend-Rework. 

**__READY TO MERGE__** :+1: 

Changes: 
- [x] Added a `config-template.ini` instead of the real `config.ini` to git
- [x] Moved cache for apps into a central directory instead of `public/<appname>/data`
- [x] Removed node_modules and added them to ignore
- [x] Reworked Movie-App
- [x] Reworked Music-App
- [x] Added a `app-cache-handler.js` that handles everything about Cache-Data of an app
- [x] Added a `configuration-handler.js` that handles everything about the `config.ini`
- [x] Reworked movie playback and metadata fetching
- [x] Code-Style fixes

Feel free to comment changes you don't like or don't understand why i made them.