Refactor inbox_block_script library

Here's one of the scenarios I've manually tested:

• On a clean install, apart from UID1, I've created 9 regular users (user1, user2, ..., user9)
• As UID1, I've created 9 threads: first with user1 message "THREAD 1", then user2 message "THREAD 2"... and so on.
• Double-checked in the backend to make sure each thread has only one message and a distinct user
• Placed the "Private Message Actions" and "Private Message Inbox" blocks inside the Sidebar region of Olivero theme
• Browse to /private-messages page
• Inbox shows correctly 5 threads. From top to bottom: THREAD 9, THREAD 8, ..., THREAD 5
• The "Load Previous" link is there
• I click "Load Previous" and expecting to append the following batch to the list (THREAD 4..1)
• But only THREAD 4 and THREAD 2 are added
• I wait a little until the Ajax URL https://private-message.ddev.site/private-message/ajax-callback/get_new_... is called
• Ajax inserts also THREAD 3 in the right place
• But THREAD 1 is still missing regadless of how many Ajax refreshes are performed

In parallel, I've installed a new instance with the current 4.x HEAD and there works as it should

Created ✨ Inbox should show the thread from URL in the list Active as a followup

Problems outlined in #5 are fixed. Approving...

Thank you!

This refactoring will not be backported in 3.0.x

Automatically closed - issue fixed for 2 weeks with no activity.