Refactor Project#after_create_default_branch

What does this MR do?

The logic for this method is moved into a separate service class. Ideally we'd get rid of Project#after_create_default_branch entirely, but unfortunately Project#after_import depends on it. This means it has to stick around until we also refactor Project#after_import.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/release/framework/issues/108

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer