R1:cd406948fec7

Don't compute a nonsensical assigned task count for logged-out users

Summary:
When a logged-out user views the home page, we currently compute a meaningless "assigned tasks" count which just counts every open task. Don't do this.

Ideally `ManiphestTaskQuery` should get some tightening up too (that is, `withOwners(array(null))` should not select every task), but that might affect other stuff and the performance implications of counting every open task are affecting WMF, so just fix the immediate issue for…