fix: Fixes #361. Restrict user from clicking "Scan" to process tx before gas is calculated

* Add `isEstimatingTxFee` so we can check if all values including `gas` are available
without actually having to calculate the tx fee and incorporate into estimateTxFee method

* Display and disable the "Checking..." button if the `isEstimatingTxFee` returns false (i.e. `gas`
still undefined)

* Note: The bug associated with #361 appears to be Parity Signer-specific. If the user can click "Scan" before the `gas` has been calculated, then when it tries to go to the paritytech/fether/packages/fether-react/src/Send/TxQrCode/TxQrCode.js, where it calls `getRlp()` to get the value for the rlp prop of the QrSigner component, which calls `transactionToRlp(this.tx)` to get the RLP of the unsigned tx that is provided as an argument paritytech/fether/packages/fether-react/src/stores/sendStore.js, it crashes if `this.tx` contains a `gas` property that's undefined.
So we need to prevent the user from being able to click "Scan" before the `gas` property has been determined.
We do this by disabling the button and displaying "Checking..." until `isEstimatingTxFee` returns true, and only then
do we display "Scan" and enable the button