Request queue management

[Dentosal]

Currently we rate limit dispensing to 20 events per minute. If the amount of request exceeds that, the rest of the requests are queued. In the current implementation, the user is not shown any indication of this, and the website simply seems to do nothing. We should at least show the user that they are in queue to receive the tokens. Since we know how long the queue is, that could be displayed as well. However, we likely shouldn't let the queue to grow too large, since the browser will timeout the request if it's waiting too long for a response, typically after 1-3 minutes.