Set a constraint on sessions to avoid multiple sessions (if feasible)

[oliche]

Unique together: subject / date / number.
Should we add type ?
Is the constraint effective on nullable number (ie. for base sessions).

[k1o0]

If type == 'Experiment' {number, date, subject} = 1;
If type == 'Base' {null, date, subject} = 1;

[rai-pranav]

Should be done REST view