Add CreateMigration action

This Thor-action isolates the logic whether to (over-)write migration and
what is shown to the user. It's modelled after Thor's CreateFile-action.

This solves the issue that removing a non-existing migration, tried to
remove the template-path (#13588).

Related issues: #12674
Merge pull request #13612 from eval/issue-13588

Add CreateMigration action