Discuss channels now open where each user last stopped reading, instead of automatically jumping to the newest message. This reduces scrolling and makes it easier for users to continue conversations, with an added banner to quickly return to the last read point.

Currently, discuss channels open at the bottom of the discussion.
This is annoying as users have to scroll up to find the last message
they read.

After this PR, channels will directly open to the last read message
of the user.

To ease channel navigation further, a banner to jump to the last read
message is added.

task-3551627

enterprise: https://github.com/odoo/enterprise/pull/63180

WhatsApp conversation channels now open directly at the user's last read message instead of an earlier or default point. This makes it faster for users to pick up conversations where they left off and reduces time spent scrolling through chat history.

task-3551627

community: https://github.com/odoo/odoo/pull/166215