Store second accuracy in manual departure/arrival

[dancojocaru2000]

As per #4061, seconds used to be stored in the timestamp, which were then removed with #3980.

I find storing the seconds as well to be valuable, since a train supposed to arrive at 13:50:00 that arrives at 13:50:54 isn't really on time, and Deutsche Bahn uses 30-second steps to measure time (including planned 30 second stops at some stations).

Even if the seconds are not to be shown in the frontend, I think it would help for them to be in the API for more accurate stats when doing aggregate statistics (such as an end-of-year overview).

I also think it wouldn't be a big issue to store them since they were stored previously and are now explicitly filtered out, so the filter would just have to be removed.