Introduce converting steps in Import/Export feature

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Description

GitLab Import/Export feature is great. You can import any GitLab projects which are compatible with targeting Import/Export version.

However, when we perform post-migrations (moving data from old structure to new structure), there is a race condition that data in old structure could be imported after migrations ran.

I think we need a converter to translate data from old structure to new structure.

Proposal

when we implement post-migrations, we implement a converter too. This converter takes an effect at the beginning of post-migrations process, and redirect/remap the hashes from old structure to new strcture.

The advantages are

• We can block data in old structure. All data go into new structure directly.
• We don't need to bump Import/Export version when we ship post-migrations
• Once we bumped Import/Export version, we can remove the current converters as it's no longer need.

/cc @grzesiek @jameslopez @ayufan