Add undo support

[hellais]

We should allow users to undo some changes they have made to a certain test list possibly with some button in the changes section.

This will also involve some changes to the backend.

[agrabeli]

Yes, +100!!!

I think this feature is super important. I personally find it frustrating not being able to undo changes I've made, because it means having to wait 24h until my login has expired in order to "undo" changes (and at that point, you need to re-do all changes from scratch).

[agrabeli]

Perhaps we can prioritize this feature after the public launch.