Feature installer bundle

- [x] Moved the installer to a bundle
- [x] Removed the first step (that only checked if spoon was available). This isn't necessary anymore since composer install is checked in the front controller.
- [x] Refactored installation to a service (for reusability)
- [x] Added assetic for asset management
- [x] Allow databases without a password (could be useful for local development)
- [x] Cleaned up installer JavaScript
- [x] Removed the need for an installed.txt file (since the parameters.yml file is only available after installation)