polish the signing event creation when using forks

[jku]

The signer tools support either pushing directly to the signing event, or using a fork and making PRs. The latter mode is a bit cumbersome as it requires explicitly creating the signing event branch first on the upstream remote.

In repository-playground I handled this by creating a branch if an issue is labeled with the signing event name ("sign/*"). I think this would work:

• When you want to make a change, creating an issue is a logical start -- but the signing tool can also check if the upstream branch exists and refuse to operate until the branch exists
• if you are a maintainer you can just label the issue when you create it. Otherwise a maintainer will have to do so. A Workflow creates the branch when the label is created
• now the signer tools can be used to create fork-branches, and those PRs will go to the signing event branch