Dogfood squash commit template for gitlab-org/gitlab-ui

In gitlab#348263 (closed), we changed the gitlab-org/gitlab project's squash commit template to %{first_multiline_commit}. For consistency, I propose we do the same for gitlab-org/gitlab-ui.

Doing this would also help avoid (but not eliminate) merges which end up not using conventional commit messages, because the MR was squash-merged, and the MR's title ends up being used by default, which may not have the right format.

If you have any objections to this, this is the place to say so!

Open questions

• If we set the default squash commit template, should we document that in the repo itself?