[WIP] Merging public calendar sharing

:exclamation: This is a rebase of https://github.com/owncloud/calendar/pull/308.

@MorrisJobke @jancborchardt @georgehrke please check this out. It is my first merge so it might not be perfect :smile: 

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These are Polish organisations, and the person best fit to speak for both them on this matter is not really fluent in English. I will be happy to work as a liaison, though. If you send me an e-mail (rysiek at fwioo.pl), we can get in touch.