Support truer alternative proposition based justifications

[carlgieringer]

It's polarity is either neutral (#33) or false. See also #528.

[carlgieringer]

I think my original intention with this feature was to support a situation like: "this proposition may be true (or false, according to the polarity of the Justification), but most people are actually interested in this other Proposition, which has the opposite truth valence." And people would upvote the Justification if they agreed.

But it's an odd type of Justification since it doesn't actually include any basis for believing a truth valence about the target Proposition. And I wonder how this feature relates to something like a "preferred phrasing Proposition relation" (#13). Would we need this feature if we had that relation? It's more of a re-use of the Justification position on the page to advertise the relation.