The active thread is incorrectly set.

Problem/Motivation

The "active-thread" class should be added to the thread that is currently displayed.

Steps to reproduce

Add the inbox block.
Add a few threads.
Navigate directly to the second thread from the inbox block by visiting /private-messages/thread-id.
The active thread is currently set to the first item in the inbox block, but it should instead be the second one.

Proposed resolution

Update js logic