perf(frontend): reduce time required to switch conversations/calls

[Antreesy]

Reference / previous discussion: #15336
Quick fix: #17782
Drawback: router listener takes longer, as leaveConversation > joinConversation are now conseqent requests to server/HPB

To verify:

If I am not mistaken (but I am speaking from memory, so take this with a pinch of salt) joining a new conversation implicitly leaves the old one, so a possible improvement might be to wait only if the user is active in a call, and otherwise just switch to the new conversation without waiting. This should solve the race condition when switching between calls and at the same time prevent the delays introduced with this fix when switching between conversations.

Originally posted by @danxuliu in #17782 (review)

• API: whether DELETE to room/{old-token}/participants/active is required, if followed by POST to room/{new-token}/participants/active within the same PHP session?
  cc @nickvergessen
• HPB: whether SIgnaling.leaveRoom is required, if followed by Signaling.joinRoom
  cc @fancycode
  

  spreed/src/utils/signaling.js

  Lines 250 to 266 in c5bebda

  	Signaling.Base.prototype.leaveRoom = function(token) {
  	this.leaveCurrentCall()
  	.then(() => {
  	this._trigger('leaveRoom', [token])
  	this._doLeaveRoom(token)
  	return new Promise((resolve, reject) => {
  	this._leaveRoomSuccess(token)
  	resolve()
  	// We left the current room.
  	if (token === this.currentRoomToken) {
  	this.currentRoomToken = null
  	this.nextcloudSessionId = null
  	}
  	})
  	})
  	}