Allow using time slot numbers in long term absences dialog (instead of datetimes)

Next to the time selects, there should be selects for lesson numbers if Lesrooster is installed. If a lesson number is selected from these selects, the corresponding time field should get the start time (if start) or the end time (if end). To provide this, there should be a GraphQL endpoint that returns the lesson numbers with their start and end times in Alsijil. If Lesrooster is installed, the endpoint returns the slots of the current validity range, if not installed an empty list. If the list is empty in the frontend, the selects shouldn't be shown.

Use if apps.is_installed("aleksis.app.lesrooster"):