Add gas estimate for sticker pack purchase.
Update transaction for sticker pack purchase.
Add GasValidator component which validates gas is selected correctly and displays an error message if not. This component is not visible until it is not valid (at which point the valdiation error message is displayed).
In a future PR, need to:
1. estimate gas for token txfer (sendTransaction) via a normalised method for estimating gas for EthSend
2. move sticker pack purchase to use an EthSend object so gas can be estimated and tx sent
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