Add validation rules on date field and study ids

The following validation rules were part of the date field issue. Opening a separate issue here. Additional validation rules:

if a study id starts with PKDB it should follow the ^PKDB\d+$ pattern
for a study with a ^PKDB\d+$ identifier the date must be set in the study.json
for studies without a ^PKDB\d+ identifier no date is allowed in the study.json, here the date field is set to the upload time on upload!

matthiaskoenig / pkdb