[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
I guess it lacks testing due to appframework integration/migration https://github.com/owncloud/core/blob/master/lib/public/appframework/http/response.php#L143