Add Configurable Max Queue Capacity Limit

[daltonfury42]

Admin can optionally configure a max capacity, after which new people will be put on hold.

This might add value in certain situations, but I am a bit reluctant, as it's not a core feature. For example, admin can also adjust without it, I feel.

[daltonfury42]

Create a new right side panel button to open a settings modal (use this as reference), in which we can add this as an optionally configurable entry.

Right now the modal would look empty with just one configuration item, but soon we will add more parameters.

To set the max queue capacity, send a patch request to /queue/{queueId}, example payload:

{
  "maxQueueCapacity": <some-number>
}

Currently we have not handled the case at backend side where we need it to be unlimited, so we can set a very high value say 100000 for now when the user wants to remove the limit.

[maaverik]

Closed by #603