Added an optional flashmessage listener

This adds an optional flash eventlistener, which shows flashes to the user when they do certain actions.

### What's included:
- the listener
- tests for the listener
- updates for the english translation file
- updates for the dutch translation file


### The current supported actions are:
- user sends a message
- user deletes a thread
- user undeletes a thread

### How to enable
```
fos_message:
    flash_messages:  //optional if user doesn't add this we just don't show flashes
        show: true  //boolean but if false user shouldn't even add this segment
        flash_key: my_custom_key //optional string  = the key name of the flash, defaults to success
```
### Todos
- Add some documentation for this

### Credits
The code is based on https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/EventListener/FlashListener.php