Cleaning up #489's implementation

[jamesbraza]

I felt my implementation in #489 was bad:

• _PAPERQA_ROOT_LOGGER was a vague name -- unclear if "root" or "paperqa"

• It's not clear the RichHandler is getting installed, so I made a flag for it

• This was unintuitive, as it's true once at the start of CLI

  if not is_running_under_cli():
      _PAPERQA_ROOT_LOGGER.addHandler(rich_handler)