FE: Max User count activity tweak graph to support UI

Background - #334689 (comment 619838538)

In https://gitlab.com/gitlab-org/gitlab-ui, tweak the line chart and add the option of a dashed line style (lineStyle.type: 'dashed', see here).

Availability & Testing

As we are altering functionality and adding another piece of it, let's ensure we have frontend tests that ensure the rendering of the line chart.

We should also add a test to check the interactivity of toggling between the two styles.