[Snapshot Restore] Disable navigation via step name when the form is invalid

Relates to #95876 fixed by PR#95939.

When a step (index settings) is invalid, it's possible to navigate to the previous step (logistics) by clicking its name in the wizard top navigation bar. That way the wizard will be blocked on the 1st step with a disabled 'next' button.

https://user-images.githubusercontent.com/6585477/116881324-6ac41400-ac23-11eb-8721-6df3210ca2d3.mp4

To reproduce:

1. start ES with a folder for snapshots by adding -E path.repo=/tmp/es-snaps to your ES start command
2. create a snapshot repository and a policy
3. run the policy
4. navigate to snapshots list and try to restore a snapshot
5. in the wizard navigate to 'index settings' step
6. enable "Modify index settings" to introduce a form error
7. navigate to the previous step by clicking 'Logistics' in the top navigation bar