Save caller in MessageEndpoint add method

[wszarmach]

PR review suggestion to save caller passed in add method. See PR #757 (diff)

   @ApiMethod(name = "contactMessages.add", path = "contact-messages", httpMethod = ApiMethod.HttpMethod.POST)
    public void add(@SuppressWarnings("unused")User caller, ContactMessage message) throws UnauthorizedException, ForbiddenException, BadRequestException, NotFoundException {