add warning to "add to study"

We got reports that users sometimes select the wrong study. Our first idea was to add either undo or a confirmation. However, the current UI is very compact and works well for what it is. For example, "create new participation" and "change existing participation to 'participating'" are currently handled the same while still being discernible.

Adding undo or confirmation would break the existing UI and add significant complexity. So we figured that it is not wort the effort, especially given the low priority of this feature.

The first sentence is taken from the documentation