Fiat amounts are now shown inline on token transfers in the transaction
list, where possible (i.e. where the conversion rates are known).
The logic for this hook is pretty tangled because it's used for so many
fundamentally different types of items (eth transactions, token
transactions, signature requests). In the future we should split these
into different components.
The documentation for the `useTokenFiatAmount` hook was updated to make
`tokenAmount` optional, but in practice it already worked as expected
without the amount being passed in.
This hook is responsible for converting a token balance to fiat. It has
been extracted from the `TokenCell` component, and will be used
elsewhere in a future PR.
