Add WIP iCal and Google Calendar imports in frontend

[magnusrodseth]

Closes: #567

Changelog

• When the user clicks on an event, the user is taken to that event's index page. The user then selects either iCal or Google Calendar from a dropdown.
• If iCal is selected, the frontend queries the backend for a .ics file.
• If Google Calendar is selected, the future plan is to send a POST request to the Google Calendar API.

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This pull request introduces 2 alerts when merging 95e4226dc9d6c318fa5f8ca14117a67f17824d1a into 16342a86a735dfc8d28a1cd474252ba6c423028a - view on LGTM.com

new alerts:

• 2 for Unused variable, import, function or class