Implement error bars on date axis

Originally reported in https://github.com/plotly/plotly.py/issues/1488

Here's an example that specifies error_x and flips between setting the xaxis.type property between 'date' and 'linear'.

CodePen: https://codepen.io/jonmmease/pen/zXrmOK

var fig = {
  "data": [
    {
      "error_x": {
        "array": [
          100000.0,
          100000.0,
          100000.0
        ],
        "type": "data",
        "visible": true
      },
      "type": "scatter",
      "x": [
        1554370371547.085,
        1554370471547.085,
        1554370571547.085
      ],
      "y": [
        6,
        10,
        2
      ]
    }
  ],
  "layout": {
    "xaxis": {
      "type": "linear"
      // "type": "date"
    }
  }
}

Plotly.newPlot('myDiv', fig, {showSendToCloud: true});

with xaxis.type: 'linear':

with xaxis.type: 'date':

Start with the CodePen reproduction and compare the plotted error bars when xaxis.type is set to 'linear' versus 'date'. Trace the date-axis and error-bar handling, then verify that error bars render correctly for the supplied date-axis example.