R1:fa4dcaa3aaff

Stabilize sorting of feed stories with similar strength

Summary:
See PHI1222. When we publish several transactions to feed at once, we sort them by "action strength" to figure out which one gets to be the title story.

This sort currently uses `msort()`, which uses `asort()`, which is not a stable sort and has inconsistent behavior across PHP versions:

{F6463721}

Switch to `msortv()`, which is a stable sort. Previously, see also T6861.

If all transactions have the same strength, we'll now consistently…