campaigns: add authz checks to user_credential store methods

Created by: LawnGnome

What @ElizabethStirling has brought up about authz on DB layer (via CheckSiteAdminOrSameUser) is something I would ask for too. It does worry me that when we have more call sites of these methods other than campaigns code, we would easily forget to do authz.

(I believe it is OK to have authz enforced on both resolvers and DB layers)

Originally posted by @unknwon in https://github.com/sourcegraph/sourcegraph/pull/15512#discussion_r519573053