[6.x] Ensure "cmd + s" bindings are destroyed when moving pages

[duncanmcclean]

This pull request fixes an issue where cmd + s keybindings weren't being destroyed/unbound properly when moving between pages.

This PR fixes it by destroying the bindings under beforeUnmount, instead of unmounted. Not entirely sure why it fixes it, but it does. 🤷‍♂️

How to reproduce

1. Visit entry publish form
2. Go to the collection's blueprint
3. Trigger cmd + s
4. See a console error, coming from the save method in the entry publish form component.