Inbox should show the thread from URL in the list

Problem/Motivation

On the /private-messages/{private_message_thread} page, when the inbox block is present, the inbox doesn't always contain the thread indicated in URL.

It's a UX expectation that the current loaded thread should be visualized also in the inbox. Frontend might want to style it differently, indicating the end-user that it's the selected thread.

Steps to reproduce

• With Olivero, place the inbox block into the "Sidebar" region
• Create 10 threads: IDs: 1..10 (where 1 is the first, 10 the last)
• Navigate to /private-messages/3
• The main page content correctly renders the thread 3
• But looking at the inbox block the thread 3 is not there. Only latest threads are shown (10..6)

Proposed resolution

When the current thread context is determinable from the URL, in inbox, load a segment of threads that contain the current thread. Otherwise fallback to the current behaviour.

Remaining tasks

Decide whether the current thread is rendered in the top of the list or at the bottom. Probably on top is more proeminent but at the bottom makes more sens because it might not have other threads after.

User interface changes

If the current thread can be determined, the inbox will "scroll" in a way that the current thread is visible.

API changes

None.

Data model changes

None.