Part of ENG-1729
Sidebars are always displayed under their parent chat, so the distinction between Focused and Background doesn't make sense and the notification dialog is misleading.

• Removes the third "Background" option
• Removes the statements of "Living in Focused/Background tab"
• Renames the title to "Thread notifications"

1. Check if the notifications dialog looks correctly for a sidebar
   • check if it works if the sidebar was set to a Background tab before the change
2. Check if it the dialog looks and works as previously for other chats