Don't assume user knows agent being `shared` with

[langsamu]

When 'sharing' (a misnomer) a resource with a WebID, after modifying the resource's ACR, the application tries to modify the authenticated user's WebID profile, asserting that the authenticated user's WebID foaf:knows the WebID that the resource has been 'shared' with.

E.g.

1. webid1 is authenticated.
2. They 'share' resource1 with webid2.
3. App PUTs webid1 adding the statement

<webid1> foaf:knows <webid2>

This is wrong.
Adding an agent to an ACR does not imply that the object is "known by" the subject, let alone any "level of reciprocated interaction between the parties".