Swift migration follow-up

* Follow up for https://github.com/nextcloud/talk-ios/pull/2561

- [ ] `activeAccount` should be an optional, it's possible no account is active
- [ ] "Logged out" message should contain the user and account, similar to `presentAccountNotConfiguredAlert`
- [ ] Double check that on logging out the active account is only changed, when it's the one that was logged out
- [ ] Check if merging `presentVoiceRoomJoinAlert` and `presentAlert(for pushNotification: NCPushNotification)` makes sense

Start with pull request 2561 and trace the code using `activeAccount`, `presentAccountNotConfiguredAlert`, `presentVoiceRoomJoinAlert`, and `presentAlert(for pushNotification: NCPushNotification)`. Review each checklist item against the logout and push-notification flows; done means all four follow-up questions are resolved and the alert behavior remains consistent.

Written by the indexing model from the issue text.