Move the logic of the antibot to a service

Thanks @hmendes, can you review it again, please?

It guess we could change this part

    $skip_antibot = FALSE;

    // Set skip antibot to true if user has appropriate permission.
    if ($this->user->hasPermission('skip antibot')) {
      $skip_antibot = TRUE;
    }

to

   $skip_antibot = $this->user->hasPermission('skip antibot');

But its not a required change, so I'll change to RTBC anyway.

Patch for 2.0.x