Use the user in request for deciding the layout for non-home DAV requests

[ishank011]

For the incoming /dav/files/userID requests, we have different namespaces depending on whether the request is for the logged-in user's namespace or not. Since in the storage drivers, we specify the layout depending only on the user whose resources are to be accessed, this fails when a user wants to access another user's namespace when the storage provider depends on the logged in user's namespace.

For example, consider the following case. The owncloud fs uses a layout {{substr 0 1 .Username}}/{{.Username}} and it's mounted at /users. The user einstein sends a request to access a resource shared with him, say /dav/files/marie/abcd, which should be allowed. However, based on the way we applied the layout, this can only be translated to /users/e/marie/abcd.

Fixes #1215

[labkode]

The code and behaviour looks good to me, this will also allow oc web to access shared resources from the tab without having to mount them. @ishank011 some acceptance test are failing in CI

[ishank011]

@labkode thanks for the quick review! That is just one test which is pretty unstable. I've retriggered the CI

[butonic]

LGTM