Add button to subscribe to calendar

[olillin]

The purpose of this PR is to add a way to subscribe to the mottagning schedule. It does this by creating a new API route /api/schema.ics which serves an iCalendar file containing the events from the calendar. It also adds a new button on the schedule page which let's you subscribe this calendar file.

Changelog

Below is a list of the changes in this pull request. The format is based on Keep a Changelog.

Added

• /api/schema.ics which server an iCalendar file containing the mottagning schedule. The file is generated by combining the calendars fetched from the _kalender[n] links. The calendar takes the name and other data from _kalender0 and appends all the events of the other calendars. The calendar is cached and expires after a set time (see below) or if the calendar URLs are changed.
• CachedCalendar class which holds a cached calendar body as a serialized string.
• the environment variable CALENDAR_CACHE_EXPIRATION_TIME which defines a time in minutes before a cached calendar will expire and need to be regenerated. The default is 3 hours.
• Modal component, used for popup windows.
• CalendarSubscribeModal, a popup for subscribing to a calendar at a certain URL, defined by a prop.
• a button on the /schema page which opens a CalendarSubscribeModal for subscribing to /api/schema.ics.
• iamcal dependency for editing calendars, used in the merging of calendars.

Changed

• the z-index of the page header from 50 to 20 so that modals can be shown on top of it.

[daancs]

lgtm