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:

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

If all transactions have the same strength, we'll now consistently pick the first one.

This probably (?) does not impact anything in the upstream, but is good from a consistency point of view.

Test Plan:
Top story was published after this change and uses the chronologically first transaction as the title story.

Bottom story was published before this change and uses the chronologically second transaction as the title story.

Both stories have two transactions with the same strength ("create" + "add reviewer").

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20540