[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
@rysiekpl can you put me in contact with those organizations? We want to improve accessibility, and especially when it’s a matter of improving the HTML, I want to do that.