Fix confusing ordering of similar actions in transaction groups

Summary:
Fixes T7250. Currently, if a display group of transactions (multiple transactions by the same author in a short period of time with no intervening comments) has several transactions of similar strength (e.g., several status change transactions) we can end up displaying them in reverse chronological order, which is confusing.

Instead, make sure transactions of the same type/strength are always in logical order.

Test Plan:

• Merged a task into another task, then reopened the merged task.
• Before patch: merge/reopen showed in wrong order.

• After patch: merge/reopen show in correct order.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7250

Differential Revision: https://secure.phabricator.com/D14680