Do not allow a user to be deleted if they are assigned in the bot

We could have a warning message before deleting the user, in case the user is assigned in the bot, and do not allow deletion until the user assignment is changed in the bot.

I made a video showing what happens. If we delete a user who is assigned in the bot, the bot continues assigning conversations to the deleted user: https://www.loom.com/share/7126d5acfe2549b9bc05c7a41b4a147c