Change status wording

Some of our status wordings are confusing, e.g. "show up" for appointments before the appointment has even taken place. "invited" for participations implicitly also means "has participated" once the study is over.

Some of this confusion stems from the fact that the interpretation changes with time. "Invited" is a perfectly valid interpretation before the subject has participated, but confusing afterwards. "Has participated" would have the same issues the other way around. As a compromise I tried to use words that make sense in the middle, i.e. during study execution.

The first commit only changes the UI strings and works on its own. However, I prefer if we use the same words when we talk to each other as when we talk to users, so I think we should align the code as well. That is what the second commit is for. The main downside is that it breaks some URLs.

I am not totally sure about the wording I picked. Let's discuss them!