Resolve private links in oCIS

[kulmann]

Description

oCIS is now capable of announcing private links in PROPFINDs. The web ui needs to be able to resolve those. Clients (e.g. Desktop, or back-buttons in external editors) need private links in order to open the files app of the web ui, resolved and navigated to the file id from the private link.

Reasoning for private links instead of internal links

While internal links achieve the same goal, the private link is constructed by string concatenation in a defined format. The internal link has to be created first. If e.g. the Desktop client would need to create an internal link backend side, only to let the user navigate to the file in the web ui, we'd create a lot of link shares that would otherwise not be needed at all.

In contrast to oc10 where some people are used to copying private links from the web ui, we don't want to show the copy button in the web ui for oCIS. There is a capability (planned, ETA today or tomorrow) to disable the button in the ui.

Expected behaviour

Resolve private links to files in any space or share.

Actual behaviour

Private links to files in the personal space do seem to resolve. Resolving files in other spaces fails with an error. (at least in #7430 )