Convert seconds to minutes and hours on chat notifations

In the pipeline message, the duration of the pipeline was shown as:

> gitlab-org/gitlab-ee: Pipeline #8002259 of master branch by Douwe Maan
failed in 8612 seconds

But this many seconds only gave me a clue how long the pipeline took,
so now this will be shown as something like: 02:20:12 which gives a much
better measure of time.