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

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 https://github.com/nextcloud/spreed/pull/17782#pullrequestreview-4161994269

• 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
  https://github.com/nextcloud/spreed/blob/c5bebdafbf44a24bd42503879f786b6bbf8a255a/src/utils/signaling.js#L250-L266

Start with the previous discussion in #15336, the quick fix in #17782, and the signaling logic in src/utils/signaling.js around lines 250-266. Investigate whether the active-participant DELETE and Signaling.leaveRoom are required before switching, then establish a verified conversation and call-switching behavior without the introduced delay.