Improve perceived display performance of projects graphs and stats on home page

[pombredanne]

It can take a long time to display the graphics at the bottom of a project page for large codebase with many packages and resources.

I think we should revisit when and how these are computed and displayed.

• we could pre-compute and cache these computation when a project scans are complete
• we could display these only on request with a click on a toggle button
• we could display these automatically for small projects (say with less than 5K resources) and display on demand otherwise

[pombredanne]

Dupe of #800