Fixup some database migrations

[horcsinbalint]

There were some ideas discussed for adding some migrations in #669 . Some them should be checked out:

• Decide if having enums in the DB has any benefits (most probably not). If not, then we should move to text fields instead
• Check if we can add and whether should add some foreign key constraints (foreignIdFor and foreignId does not create foreign keys)
• Move the id of semesters table to unsignedBigInteger from unsignedSmallInteger (there is not much benefit of using small integer in this case IMO and diverging from standard practices can make our life more difficult - I had to debug an error caused by this today)