Move covmanager graph to d3

[jschwartzentruber]

Context:

All other graphs in FuzzManager use d3. Covmanager is the only component left that still uses chartjs. For maintenance purposes, it would be best to migrate covmanager to also use d3.

Requirements:

Covmanager charts using equivalent visualizations in d3.

Acceptance Criteria:

Covmanager graphs use d3, not chartjs. Graphs convey same information as before.

Additional Notes:

Include any extra information such as constraints, assumptions, or risks.

[gitstart-connector]

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[gitstart-connector]

This issue has been handed off to GitStart: https://clients.gitstart.com/mozilla/12016/tickets/FUZ-12

[jschwartzentruber]

Fixed in #17