Add check for BRL ramping amount

[ebma]

• Refactor code for getting subaccount data in the frontend. Move the logic to a separate function and make the checks for missing user or invalid kyc more expressive.
• Add new endpoint for retrieving user limits to the signer service.
• Add logic to query the user limit from the signer service. Note that this endpoint requires the subaccountId instead of a taxId. I decided to leave this coherent to the BRLA API as we have access to both in the frontend when doing the query.

Closes #518.

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[ebma]

Superseded by #597.