would not reinvite

This adds a new field Participation.would_not_reinvite which is planned to replace Appointment.show_up. There are many open questions though:

• Default value: IMHO it makes sense to assume that everythink is good, but some users expressed the wish to more explicit. We could use null as a default value
• double negative: On the one hand, boolean options should default to False. On the other hand, boolean variables should have a positive name to avoid double negatives. I broke the latter rule here.
• Wording: Both the name of the field as well as the help text
• UI: I wasn't sure where to add the checkbox. The only place that made some sense was the view where you can enter a subject's news interest. (I was also thinking about merging the dropout view into this, but that had some technical difficulties.)

Things I did not yet tackle

• migrating from the old data (proposal: would_not_reinvite = showup_score > 5)
• aggregate the values from all participations into a single "trafic lights" score
• replacing showup by this new mechanism (see also !1877 (closed))