* Remove titleOffset and add placeholder width for header nav buttons

* Remove passing `screen` prop for specific pages

* Change Account screen so its header uses the Information component with a big identicon, a short address,
so it is clickable, and with identicon on left of name and address.

* Apply extra bottom margin between name and account address when Information component used in the header

* Update header so it uses Flexbox according to docs as intended

* Since we're using Flexbox without placeholder spacing for the left and right header buttons, when the title
is shown in the header it is offset if there is only a left or right button. So a `titleOffset` prop
is used on pages where the Header component is used to indicate if the title needs to be offset to the left
or right so it appears centered in the screen. We also pass a `screen` prop (named after the component where the
Header component is being used so we can tailor the header spacing on specific screens.

* Re-order props alphabetically

* QUESTION - should we consider always adding placeholder spacing for where both right and left buttons in header would be as an alternative so no offset for the title is require?

- Add option to import a Parity Signer account. It is then stored in
local storage.

- Send transactions with Parity Signer accounts

- Refactor sendStore so that "token" (and chainId) is inside this.tx;
avoids having to pass `token` around every function, and it makes sense
to put it in the object since it is part of the tx info.

- Add account type (either node or signer) to accountsInfo

- Add account type to withAccount; output props are now `{account:
{address, type}}` instead of `{accountAddress}`

* Move toggle details buttons and associated methods and showDetails state into nav of TxForm component

* Pass showDetails down as props to TxDetails

* Remove previous attempt to display TxDetails like a popup above other fields
(i.e. remove form-details-wrapper and -details-value styles)

* Move "Details" / "Hide" buttons into the form-nav footer section

* TxDetails now appears above the footer area above other input fields without causing the screen height to increase

* Background of the TxDetails darkened so it stands out from surroudings

* Change "Details" arrow to point upwards since TxDetails now shown above

* Change "Hide" arrow to point downward since TxDetails now shown above

* Rename classnames to `_` or `-` more consistently

* Note: To quickly display the 'Details' button so you can click it change TxForm.js as follows:

```
{/* {valid && this.estimatedTxFee(values) ? ( */}
  <TxDetails
    // estimatedTxFee={this.estimatedTxFee(values)}
    estimatedTxFee={new BigNumber('8100000')}
    token={token}
    // values={values}
    values={ { amount: new BigNumber('0.1'), gasPrice: new BigNumber('21000') } }
  />
{/* ) : null} */}

* Remove `isEstimatedTxFee` since can just use `estimatedTxFee` to achieve same functionality

* Rename `newEstimatedTxFee` to `newGasEstimate` since it returns a value that's used for the `values.gas` prop

* Update `estimatedTxFee` to return `null` if the amount, gas, gasPrice values provided aren't valid, instead of returning the calculated estimated tx fee

* Wrap call to `estimateGas` in a try/catch block that throws error "Unable to estimate gas" when error thrown when calling `estimateGas` in TxForm.js

* Move check `if (!ethBalance) {` earlier in function

* Create TxDetails component and move relevant instance methods from TxForm into TxDetails

* Fix linting warning `Using target="_blank" without rel="noopener noreferrer" is a security risk: see https://mathiasbynens.github.io/rel-noopener`

* Remove isCancelled

* Remove estimatedTxFee from state

* Sort instance methods into alphabetical order

* Refactor by adding isEstimatedTxFee to check gas properties defined on form values

* Add instance method estimatedTxFee to return calculation based on gas values

* Move functions out of render function and change them into instance methods

* Remove duplicate check for whether values.gas existed

* Fixes error `Uncaught TypeError: Cannot read property 'resolve' of null at flush` that is generated after both valid amount and recipient values are entered

* Switches to font size that fits a value as small as 1 Wei on the screen when more than 9 digits entered

* Switches back to default font size when amount of digits entered is 9 or less again

* Caters for when user unchecks the Max Value button