Add visualization ground truth & contributions evolution

[xavierfav]

As part as the monitoring tools, some graphs showing the evolution of ground truth and contributions would be useful.

I plan to take advantage of this example from Freesound.

[xavierfav]

After doing a bit more research, this example seems really nice.
More generally, it comes from the NVD3 library based one d3.js.

[xavierfav]

There is also this library used in AcousticBrainz.

[xavierfav]

Highchart was used to create two graphs:

• one displaying the number of user votes (absolute and cumulative)
• a second for the number of ground truth annotations (absolute and cumulative)

For now they are in the private monitor page. Maybe it would be interesting to put them somewhere publicly on the platform. For instance, allowing contributors to see the number of votes may motivate them.
Moreover, we could draw a curve for each of the top contributors of the week (related to #8).