Allow editing commit messages when merging a stack

Generally loving all the new stack features, thank you! To make it more useful, it would be great if when merging a stack of PRs from the website if we were prompted to choose a commit message like we are if merging one PR at a time.

For further details, we require squash merges and I have a coworker who performs a ton of tiny commits per branch in the stack. The default commit message for the squash merges includes a separate line per commit + blank lines around each; this ends up as a LOT of vertical space in the standard `git log` output that is really unhelpful.

Start at the website flow that merges a stack of PRs and compare it with the existing single-PR merge prompt. The work is done when stack merges allow an editable commit message, including squash merges, while preserving the current behavior for individual PRs and avoiding the verbose default message described in the issue.

Written by the indexing model from the issue text.