Add contribution metrics for Core Performance

[westonruter]

On the ​People contributing to Core Performance page, there is currently no data being shown.

There are two sources for where contributions can be tracked:

1. Trac tickets and related commits for the performance focus.
2. GitHub activity for the ​wordpress/performance repo on GitHub.
3. Posts to Make/Core with the #core-performance tag: ​https://make.wordpress.org/core/tag/core-performance/
4. Posts to the Make/Performance blog: ​https://make.wordpress.org/performance/