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)

during save study is automatically injected into model

Test coverage unit tests

See merge request NCER-PD/scheduling-system!100

separate patient data from study data

Closes #181

See merge request NCER-PD/scheduling-system!99