* 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} */}

review-fix: Hide vertical overflow on body element so scrollbar not flicker on/off when TxDetails toggled

* 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`

* After entering a valid value and a recipient address, it works but the console displays error: `Uncaught TypeError: Cannot read property 'resolve' of null at flush (index.js:62)`

* On the 'Send THIBCoin' if you enter a valid value and a recipient address, the Tx Fee Estimate is shown as being the same value as the Amount, which is incorrect!

todo:
* add option to export json keyfile to backup
* clean up duplicated code in store