Resolve "Subjects without visit list"

Closes #176

See merge request NCER-PD/scheduling-system!106

Resolve "Subject required contact list"

Closes #177

See merge request NCER-PD/scheduling-system!105

Resolve "Notifications should be customized by study"

Closes #191

See merge request NCER-PD/scheduling-system!104

Resolve "subject should have social security number"

Closes #185

See merge request NCER-PD/scheduling-system!103

Resolve "list of subjects should contain columns dependent on the study"

Closes #184

See merge request NCER-PD/scheduling-system!101

datetime format is encoded (instead of pure ASCII characters transformed into string)

Closes #194

See merge request NCER-PD/scheduling-system!102

Merge branch '184-list-of-subjects-should-contain-columns-dependent-on-the-study' of https://git-r3lab.uni.lu/NCER-PD/scheduling-system into 184-list-of-subjects-should-contain-columns-dependent-on-the-study

there is singleton study columns object created in db to reflect current state of the project (single study per smasch instance)

during save study is automatically injected into model

there is singleton study columns object created in db to reflect current state of the project (single study per smasch instance)