Refactor prettyTime.parseSeconds to support different day/week lengths

The following discussion from !1610 (merged) should be addressed:

• @to1ne started a discussion: (+4 comments)

  @stanhu I think we should avoid parseSeconds, because it is built for 8h/day & 5d/week calculations: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/assets/javascripts/lib/utils/pretty_time.js#L16-17

  Maybe you can fit the fix in together with !2681 (merged)?