FIX: Unassign buttons on the activity/assigned page don't work

[AndrewPrigorshnev]

We've noticed problems when unassigning a topic on the activity/assigned page using this button:

One of two incorrect things happens when pressing this button:

1. The first topic on the page gets unassigned instead of the selected one
2. No topics get unassigned (that happens when the first topic on the list is not assigned and only has assigned posts)

That happens because this handler for all buttons somehow always use the same state - the state of the first topic on the page:

discourse-assign/assets/javascripts/discourse/components/assign-actions-dropdown.js

Lines 55 to 68 in f2906e0

	onSelect(id) {
	switch (id) {
	case "unassign":
	this.unassign(this.topic.id);
	break;
	case "reassign":
	this.reassign(this.topic, this.assignee);
	break;
	}
	const postId = id.match(/unassign_post_(\d+)/)?.[1];
	if (postId) {
	this.unassign(postId, "Post");
	}
	},

There seem to be some nuances in select-kit and I'm not sure whether this should be qualified as a bug in select-kit. However, I've noticed that in other cases when we use DropdownSelectBoxComponent we use the onChange handler rather than onSelect.

So I switched this code to using onChange and that resolved the issue, the onChange handler has access to correct state.

Tests will be in a follow-up.

[jjaffeux]

onSelect is supposed to be deprecated, I guess it didn't get caught.