Rebuild: Backend

Currently, the Flow Backend is a CRUD API for file management. It can ask the FF! where the configurations are located, so it doesn't need to be configured. This introduces some limitations, tho. This is only really available when a configuration is being used with the `FileSystemClassLoader`.

# Tasks
## Base
_The basics_
- [x] (Re)Connect to the FF! Management Gateway with Hazelcast
- [ ] Create an endpoint for information about the connection and health of the FF!

## Authentication
_Is the user allowed to do some things?_
- [ ] Create a way to log in
- [x] Authorize with the FF! Management Bus
- [ ] Endpoint for possible actions and endpoints

## Configurations management
_Create, Read, Update, Delete_
- [ ] Create an interface for doing CRUD actions on a Configuration
- [ ] Implement interface for executing actions on local file system
- [ ] Implement interface for executing actions over the FF! Management Gateway (Dependent on FF!)