Add relative timeseries

Add a relative timeline which looks like the absolute one but shows
per-step survival rates and makes it easier to glimpse trends.

Also rename the old "relative" to "overview" and make some tiny
cosmetic fixes.

Change-Id: I72fa2d446927f3ddcf00f3f0f25ea14163320123
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/862