A source is unique for an institute/year
Working a little more in DisciplinesSubject

There should be only one such discipline/subject pair, be it main or
not. I don’t know how to represent that a subject should have only one
main discipline as true…

Adding DisciplinesSubject to the admin (it is appearing only in the
adding of a subject/discipline, but not for edition I don’t know
exactly why)
Some placeholders views
Creating some initial views

Views for the following were created:
* discipline
* subject
* index (points to the list of disciplines)