algo: Compute trusts on a per video basis

[lfaucon]

Motivation

Trust is quickly diluted when a users trusts multiple other users. We could aleviate this by transfering more trust when only a few of the trustees have expressed ratings on a given entity. In this manner we would preserve the security guarantees

[lfaucon]

Maybe a mitigation to limit influence of trust may be that trustee receive less trust when they vote on the same entity as the trust-giver