[lib] update threadLabel to handle thick threads

Summary: issue: ENG-9173

Test Plan:
Tested that when creating a chat:

• if a chat for the selected list of users exists and is thin, proper label for the thin tye is shown
• if the chat for the selected list of users doesn't exist - "Local DM" is shown, with a lock icon

Tested that in thread settings:

• if thread is thick, "Local DM" is shown, with a lock icon
• if thread is thick sidebar, "Thread" with the align-right icon is shown
• if thread is thin, proper label and icon are shown

Reviewers: tomek, kamil, ashoat

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13333