Implement contribution graph

[flodolo]

Reference: specs

This issue tracks the implementation of the contribution graph.

Blocked by:

• Add filters for contribution graph #2485
• Implement new design layout (without contribution graph) #2486

[mathjazz]

We should consider #2138 when implementing the bottom part of the widget that displays detailed data of the selected time range. We should be able to reuse logic, data model or UI.

[mathjazz]

Adding a link to the old and now obsolete proposal for the review dashboard. It has some nice ideas (like diff) and I like that it also includes comments.

If we want the "Reviews received" view to serve as the review report, it must also include comments. I thought it's worth mentioning here since we didn't mention it in the spec.