Add reminder for DCFs that have not been filled in/completed

Well, as the title says, it would be nice to have some way to remind associations automatically that they still have a DCF hat is not completed or not filled in at all. Currently this has to be done by hand, which is not a big problem, but does take some time, especially if people need to be poked multiple times.

A simple (daily?) reminder mail that fires on events marked as drinks, which are not yet completed/filled in and past their end time would already suffice. In essence people should just complete the DCF as soon as a drink ends so I don't think a grace period would even be needed.

Start by tracing how drink events, DCF completion state, and reminder emails are represented and scheduled. Confirm how the system identifies past, incomplete DCFs and where existing mail is sent. Done means an automatic daily reminder reaches the relevant association for each past drink whose DCF is missing or incomplete, without requiring manual follow-up.

Written by the indexing model from the issue text.