Sessions hierarchy UX - tracker issue

[romalpani]

Initial work to address Session and Chats hierarchy and relationships is done here.
#332922

Follow-up items:

1. Update nomenclature of "Chats" and "Sessions" to be simpler and consistent. #333064
2. Add "+" action at a session list item, for users to create sub-sessions (aka chats) under #333065
3. Don't show sub-sessions (aka chats) as tabs in the session pane. #333066
4. Improve look and feel of connectors in the session list #333067