refactor(NODLMigration): prepare to create GrantsMigration

[aliXsed]

This a refactor not changing any behaviour according to the tests. I needed this to reuse bridge common functions for grants without duplicating or changing them.

[ETeissonniere]

Refactoring looks good, though two points of considerations due to memory layout changes (namely the introduction of the ProposalStatus struct):

1. CheckBridge.s.sol will no longer work with the current bridge
2. MigrationNFT contract wouldn't be redeployable with the current bridge

I wonder if it would make sense to try and maintain backward compatibility with the old storage layout as to keep both of these contract/script usable?

[aliXsed]

Refactoring looks good, though two points of considerations due to memory layout changes (namely the introduction of the ProposalStatus struct):

1. CheckBridge.s.sol will no longer work with the current bridge
2. MigrationNFT contract wouldn't be redeployable with the current bridge

I wonder if it would make sense to try and maintain backward compatibility with the old storage layout as to keep both of these contract/script usable?

@ETeissonniere very good point. I have now changed it to avoid any modifications of the storage layout.