Bugfix: Fix VFS enable/disable checkbox in the folder wizard

After unchecking the VFS checkbox on the last page of the folder wizard,
it could not be re-checked. The selective-sync chooser is now correctly
disabled when VFS is forced on, and the selective-sync page is skipped
entirely in that case to avoid confusion.

https://github.com/owncloud/client/pull/12144
https://github.com/owncloud/client/pull/12305
