[WIP] Add server<->server sharing

Add support for adding public links from someone to your own owncloud

Fixes #7221
This is mostly to find out whether the server that the user has entered in the input box exists.
If the user enters "myserver.mydomain.com" it will try and find whether the server exists and whether the protocol is http and https. So it's mostly a usability feature.

If we'd redirect directly to a mistyped URL there is a risk of showing an browser error page which is not that friendly. Also it means the user would need to hit the back button and retype the full URL again.