[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: 

- [ ] fix »Share by link« action + icon
Ewww, why is https://github.com/owncloud/core/blob/master/lib/private/response.php#L56 not public? :-/

Might be a good idea to migrate it. It's used by tons of apps...