Disable vanishing option when logged out

Another follow up to #6098 (account vanishing).

Problem: when the user is using the app without an account, it is confusing to be able to select Vanish Account in the Settings screen.

When the user is logged out in the app, I think it makes sense to disable the vanishing option, in a similar way how some other setting entries are disabled when logged out. (Text in grey in the UI)