Mint: Limits and balance views per keyset

[callebtc]

Limit amounts (max_balance, max_mint, max_melt) all assume the sat unit and do not work well for a multi-unit mint. We would have to add limits per each supported unit or come up with a better configuration. Checks should be refactored so they're not repeated in the code.

The balance views in the db should show issued vs redeemed = balance views for each keyset in the mint instead of a total cumulative balance that doesn't reflect reality for a multi-unit mint.

[lollerfirst]

Should new migrations be created for this or do we modify the existing ones create_balance_views etc...

[lollerfirst]

I think we can get away with just modifying the create_balance_views function.