Turn appointment validation into warnings

Fixes #148 (closed)

Most of our warnings show up every time you visit a view. In this case a study may intentionally have overlapping appointments, so showing these warnings all the time would just be annoying. As a compromise, the warnings are shown after submit, but not when you make a GET request to the page.

An issue with the resource overlap warnings (and also with the old error messages) is that users can not easily find out which time slots for a resource a free. This is not a permission issue -- user can for example get this information from the resource feed (without personal information). We did not find a good UI for this though (see !1368 (closed) and !1369 (closed)).