Set webcal as protocol and second link for webcal with events.

Related to #80 (closed)

What does this MR do?

The hidden section https://foodsharing.de/?page=settings&sub=calendar was addressed in the product working group.

Question: Whether chats, wall boards or store descriptions would be read less. Unanimous agreement that this has rather a better reliability than disadvantages. Jörg suggested:

1. to change the URL from http/s:// to webcal://, so that this is directly recognized by end devices as a calendar.
2. Possibility with the parameter "&opts=se" to include events in addition to the pick-ups. I have included these with an additional link.

How confident are you it won't break things if deployed?

(be honest!)

Links to related issues

https://beta.foodsharing.de/?page=bezirk&bid=2296&sub=forum&tid=115911 (product workgroup)

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Login as foodsaver
3. ...

Screenshots (if applicable)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
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• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
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