[UI][Dashboard] customize responsive layout

[brylie]

Story

As an API manager,
I want to change the layout of the dashboard
so that I can decide relative proportions and positions of dashboard objects.

Example

I have a dashboard with three objects:

• Map of requests
• Drill-down table
• Line chart of requests

I decide that, in a certain context, I want the map to be twice as big as the other elements, with the other two elements on a row below the map. I resize the map to fill the first row and move the other objects to a second row.

Responsive layout

Ideally, this layout will be responsive to small screens, and render objects above one another on smaller viewports.

[brylie]

Parent issue: #62

[brylie]

As mentioned in a different issue, it would be nice if the user had great flexibility with regards to creating custom layouts:
http://keen.github.io/dashboards/layouts/

[brylie]

Also, when widgets are added/removed/re-arranged on the desired layout, it would be important that the general form of the layout remain in-tact. I.e. if the user had created specific layout, such as the 'Hero Thirds', it would be important that changes to the dashboard objects do not affect the general form of the layout.