there is possibility to define columsn that are not sortable

but no sorting filtering is possible

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