conversationrepository: fix isValidUserAtCommit

The method was bugguy, because the whole point here is to check
if the conversation contains enough informations at this point to
validate the user, not to compare with pinned certificate.
Moreover, a user can sync the history from another device, without
ever connecting to the original author, so the certificateStore
will not have the device certificate in this case, so only uses
from the repository.
A test is added to reflect this.

Change-Id: I3af5e7769174eedcb54e17181d4530593960c9c9