fix: replace `visualMap` in heatmap with gitlab-ui legend component

Laura Montemayorrequested to merge
275-heatmap-update-legends-to-match-design into master

Updates legend to use our own component instead of visualMap.

Screen_Shot_2019-07-17_at_7.06.06_PM

Closes #275 (closed)

Edited by Laura Montemayor

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