feat: save lectures to device calendar

[athoscf]

Closes #1382

This implements the feature of saving all of the semester's lectures directly to the device's calendar (Google calendar, Apple calendar, etc).
Currently, the buttons to fetch the calendars and save the lectures are on the schedule page. You have to fetch the calendars first, before saving the lectures. There is no way of choosing the calendar for now, its hardcoded to use the last calendar on the list (Google calendar on my emulator).

TO DO:

• Implement calendar selection (use modal pr 1449)
• Change Exams' page to use the same device calendar library (one at a time, select calendar before adding event)

Review checklist

• Terms and conditions reflect the current change
• Contains enough appropriate tests
• If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• Properly adds an entry in changelog.md with the change
• If PR includes UI updates/additions, its description has screenshots
• Behavior is as expected
• Clean, well-structured code