Feat(Safe Apps): make chains a model with chain_id and name

[katspaugh]

What it solves

Resolves #984

• Add a new model Chain with chain_id and name
• Display it in the admin as a mutliselect

Migrations:

• Pre-populate the Chains table with the chains we support so far
• Migrate the existing data from the chain_ids field to chains
• Remove the chain_ids field

[github-actions]

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I have read the CLA Document and I hereby sign the CLA