Resolve "GitLab Migration - Wiki repositories with Projects"

What does this MR do and why?

This feature is part of the new Project Migration feature which directly migrates projects between Gitlab instances. It is currently enabled with the bulk_import_projects feature flag. This MR adds wiki migration to any existing project migration

Screenshots or screen recordings

wiki_migration_example

How to set up and validate locally

1. Feature.enable(:bulk_import)
2. Feature.enable(:bulk_import_projects)
3. Create a top level group and project in it
4. Create a wiki page in the new project
5. Go to /groups/new#import-group-pane page and enter instance url and access token (needs to be api and read_repository scope)
6. Select newly created group and click Import
7. Wait for Group import to complete and verify imported project's wiki page and contents

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #339400 (closed)