Add snippet migration rake tasks

What does this MR do?

This MR adds two rake tasks to migrate snippets to git manually:

• gitlab:snippets:migrate: Migrates snippets passing an array of ids.
• gitlab:snippets:migration_status: Shows if any migration is running in the background.
• gitlab:snippets:list_non_migrated: Shows a list of snippet ids that are not migrated.

Refs #215861 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content