Productivity Analytics: Fix scatterplot chart performance

Currently we have a big performance problem with the scatterplot chart in productivity analytics. For example, if you attempt to select a Project, or select more than 7 days for GitLab.org group, your browser will freeze. As a result we will implement a couple of solutions:

Improve sorting by [@pshutsin to fill in] in order to improve median calculation

EDITED same day by @djensen: Changed title and description