Config to display in thread buffer status bar all tags present in a thread, or tags common to all messages in thread

Allows to add **{thread_tags}** or/and **{intersection_tags}** to the Thread Buffer statusbar.

- **thread_tags**: shows all tags present in the current thread.
- **intersection_tags**: shows tags common to all messages in the current thread.

**(New P.R. with squashed commits)**