Implement symfony json_login as JSON Authentication Endpoint for api authentification

[Onyxmoon]

The user is supposed to identify himself via a POST request with a JSON body containing the credentials. As return a httpOnly cookie is to be used, which from then on should be sent back by the client to confirm the session.

This is to be done with regard to the fact that httpOnly cookies cannot be accessed via JavaScript.
This type of cookie is automatically sent with every request, but is not readable in JavaScript, which makes it safe from theft by other JavaScript.

[Onyxmoon]

Added by f221baf
Fixed by a506b9c