Add the ability to "undo" a stack

[zkamvar]

There is currently no way to "undo" or "unstack" a stack and disassociate the members of a stack. I have accidentally created a stack and found myself unable to undo it, which meant that I needed to close those PRs and re-open them individually.

It would be nice to have an undo feature.

[wackerow]

@zkamvar Have you tried this?

Top of PR:

Click the stack icon with N/M counts, then:

Click the X stack icon:

[zkamvar]

Oh okay, yeah I totally did not see that at all.