submit: detect navigation comment in imported CRs

[abhinav]

When importing CRs not currently known to git-spice, detect any stack navigation comments on the CR and import them as well.

To reduce risk of hijacking unknown comments, the comments must have both the following pieces:

• "This change is part of the following stack:"
• <!-- gs:navigation comment -->

The latter is new and placed at the bottom of stack navigation comments starting this version.

If both pieces are present, and we have permission to edit that comment, the comment is imported into the data store and updated as needed.

Resolves #325

[abhinav]

This change is part of the following stack:

• 359 ◀

_{Change managed by git-spice.}

[abhinav]

New docs: