feat(currency): add GBP support

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Someone was requesting this in Reddit

I think it'd be prudent to look at making a currencies section of settings so you can choose what currencies should be available in the balance overview toggle.

The coinmarketcap API supports a huge range of currencies.

The only problem I getting there was with making the currency choice persistent to the wallet, I have it all functioning here with the groundwork setup: https://github.com/kennedybaird/nano-wallet-desktop/tree/multi-currency

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Closing this as working on multi currency support