[FEATURE] Dialog for new users

[EmmaTellblom]

Is your feature request related to a problem? Please describe.
New users do not always understand where to find information about how the app works.

Describe the solution you'd like
Create a new dialog shown when there are no periods in the database, we already have a variable for this in calendarscreen.
This dialog is therefore a one time thing for new users. Describe that predictions are added after two cycles and inform about the "about" button in the settings.
Also add that the app requires at least two cycles in the about section.