Running times are wrong

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Running times for stages are usually displayed as 23:56:44, which are obviously wrong.

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Some context about this: we had a bug about times being weird, which was "fixed" by 9792824 ("reverted" in 3093d95).
A posteriori we realized that the running times were weird because some finished was on a different day but at a previous time than the started, so since the running time are capped to 24h in the visualization, we get weird times. So everything is fine if the difference between started and finished attributes of the Pipeline is less than 24h.

We could fix this in several ways:

1. Display something different if the difference is more than 24h, e.g. branching here
2. (my favorite) Instead of just diffing the started and finished (which don't actually represent the running time), we should sum all the jobs running times (with the assumption that the sum of the duration of jobs would practically never go over 24h)