pretty print figure json

[will-moore]

This PR pretty-prints the json when saving a figure file, so that the
json in the file is human readable and more amenable to version control (since it's not all on one line).

See https://trello.com/c/sQw7eU4K/15-pretty-print-json-when-saving-to-file

To test:
- Save a figure file and note the file-annotation ID (in figure URL)
- Get the original file ID: select file from annotation where id=32052
- Check that the file is nicely formatted - Can download the file after manually attaching the figure file to an Image in the clients OR find the original file ID and go to /webclient/get_original_file/{id}/
- Refresh the figure page to check that the file still loads correctly.

[waxenegger]

Attached manually and downloaded thereafter. Json looks nicely formatted:

{
  "figureName": "Figure_2017-5-1_20:25:41",
  "height_mm": 297,
  "legend": "",
  "legend_collapsed": true,
  "orientation": "vertical",
  "page_col_count": 1,
  "page_count": 1,
  "page_size": "A4",
  "panels": [
    {
      "channels": [
        {
          "active": true,
          "color": "0000FF",
          "emissionWave": 470,
          "label": "CFP-JP4",
          "reverseIntensity": true,
          "window": {
            "end": 197,
            "max": 197,
            "min": 103,
            "start": 103
          }
        },
        {
          "active": true,
          "color": "FF0000",
          "emissionWave": 617,
          "label": "RD-TR-PE",
          "reverseIntensity": false,
          "window": {
            "end": 124,
            "max": 124,
            "min": 96,
            "start": 96
          }
        }
      ],
      "datasetId": 1457,
      "datasetName": "archivedv",
      "deltaT": [],
      "dx": 0,
      "dy": 0,
      "height": 512,
      "imageId": 9443,
      "labels": [
        {
          "color": "000000",
          "position": "top",
          "size": 12,
          "text": "[google](http://google.com)"
        }
      ],
      "name": "/home/hudson/for_import_to_howe/archived-imoprt-case/user-1/private-1/no_projects/archivedv/CFPNEAT01_R3D.dv.log",
      "orig_height": 512,
      "orig_width": 512,
      "pixel_size_x": 0.12698,
      "pixel_size_x_symbol": "\u00b5m",
      "pixel_size_x_unit": "MICROMETER",
      "pixel_size_y": 0.12698,
      "rotation": 0,
      "selected": true,
      "shapes": [
        {
          "id": 308801,
          "radiusX": 82.7961657297979,
          "radiusY": 37.95723529794617,
          "rotation": 0,
          "strokeColor": "#0032e5",
          "strokeWidth": 6,
          "type": "Ellipse",
          "x": 202.90019790656572,
          "y": 73.80268984340069
        },
        {
          "id": -6036535494419957,
          "strokeColor": "#0032e5",
          "strokeWidth": 6,
          "type": "Arrow",
          "x1": 111.24363636363637,
          "x2": 248.08727272727273,
          "y1": 173.14909090909092,
          "y2": 40.02909090909091
        }
      ],
      "sizeT": 1,
      "sizeZ": 29,
      "theT": 0,
      "theZ": 14,
      "width": 512,
      "x": 41.5,
      "y": 165,
      "zoom": 100
    }
  ],
  "paper_height": 842,
  "paper_spacing": 50,
  "paper_width": 595,
  "version": 2,
  "width_mm": 210
}

Refresh of figure page showed no errors either.