Allow linking to stacks, e.g. /org/repo/stack/1

[Cameronjpr]

Firstly, thanks for stacks, my team is loving them!

I would find it easier to share and talk about stacks with my team if I could point them at a single link that shows the PRs in a stack. Currently I've been copy/pasting all the PR links to convey that it's a stack (e.g. in a numbered list), but it would be great if we had a way of linking to stacks directly.

Additionally, if we could add a stack description, that would be even better. I often front-load the first PR in a stack with information about the stack, but it really belongs at one level above that, as a description for the stack itself, not just the first PR.

[SethAngell]

Definitely agree with that, I've defaulted to sending the base pr and referencing that it's a stack. But a proper url would be great