Allow for custom grid configurations

[Chris Kiehl]

Currently the board is hard coded to use a 6x4 grid. You should be able to specify your own grid size although we should probably enforce a 1.5:1 ratio to keep widgets compatible.

[Iván Loire]

This was done a while ago.

You can use gridSize in your dashboard json file:

  "layout": {
    "title": false,
    "gridSize" : { "columns" : 20, "rows" : 12 },