Change column name 'period_id' to 'cycle_id' in table periods

[EmmaTellblom]

We are going to be extending our database to also be able to track predictions. With this it would also be beneficial to add which cycle an ovulation (actual and predicted) belongs to. Same with period predictions.

Period_id was not a good name to start with, it is a cycle_id in function.

This column will then be a foreign key to the tables for Ovulation_predictions and period_predictions

Make sure all SQL-queries are updated also.