Make lsst.log outputs appear in Jupyter notebook cells

Messages sent to lsst.log are not visible from Jupyter, because Jupyter only looks at Python's stderr and stdout, which are not the same as system stderr and stdout.

Following some discussion on Slack, the best approach appears to be writing an lsst.log and/or log4cxx handler that writes to either Python's stderr or stdout or some Jupyter cell output hook; the latter in particular could enable things like using rich-text output to highlight messages at different levels.