Enable giving Stacks names

[lizziemac]

I think that a GH stack should have an overall name to be identified with, as stacks are a group of changes all related to one goal. It would be even better if, in addition to #210, in the PR list these stacks could be collapsed under that stack name (I now see this is already requested in #234). This overall name could be used to provide an epic or story ID (that eventually could be linked, like JIRA already does for PRs).

e.g.
📦 [▾] Stack: [PROJ-123] Implement New Checkout Flow
↳ 🟢 #105 Add payment processing
↳ 🟢 #104 Build checkout UI
↳ 🟢 #103 Add cart state management
🟢 #102 Fix typo in header (unrelated PR)