Slave Unlink

[vincentbavitz]

Improve unlinking for slave devices.
Further refinements will mend #1111 in a future PR.

[Bilb]

I am wondering if that would not be better to keep it as a "Delete account". it's just weird to click on the "Unlink Device" but once you come back to your primary device you will still see your secondary device there.

[neuroscr]

Well the show devices screen can verify against the file server (for now) before displaying.

Which also means they need to be clearing their devicemap when unpairing

[vincentbavitz]

I am wondering if that would not be better to keep it as a "Delete account". it's just weird to click on the "Unlink Device" but once you come back to your primary device you will still see your secondary device there.

This could be misinterpreted as deleting the entire account; along with Master.
Plus we want to be able to, in future, unlink completely from secondary device.