change year_in_studies to first_study_year

[ScDor]

Is your feature request related to a problem? Please describe.
To deduct the relevant track pk (with start year), we need to know when the student started. different years have different requirements.

Describe the solution you'd like
Replace year_in_studies with first_study_year.
Note: FE should show either "I started studying on October ---" OR show 2018-19 and actually save 2018 to DB.