Ability to include stack navigation links in PR description

[abhinav]

We should provide the ability for users to opt into including the stack navigation links in their PR description instead/in addition to a stack navigation comment.
It's often useful to retain that the commit was part of a stack once merged. (Folks squash-merging will use the PR description more often than not.)

UX-wise, this probably takes the form of a <!-- gs:stack --> in the PR description.
If found on its own line, we will generate or update a stack navigation visualization under it.
We'll want to be careful about the "update" part to only match if the thing following the HTML comment is a Markdown list.

This should probably be opt-in because to avoid the additional extra API calls.

Depends on #331