Add database checks

[wikku]

The timetable app assumes some things about the database that aren't stated or checked anywhere,
like a teacher can have only one lesson at a time, Period numbers should be consecutive integers

[opti0]

Things that should be checked:

• period number as consecutive integers >=0
• teacher can have only one lesson at a time
• no or one lesson in room at a time
• an absent class cannot have a substitute