This removes some duplicated code. It doesn't appear to improve
performance at all.

It also means that you can once again use the planalyzer in a web
browser. It's a little fiddly, but you run a command like this:

       python3 ~/.local/lib/python3.7/site-packages/planalyze/backend/localserver.py  ~/.local/share/planalyzer/frontend/ ~/.cache/planalyze/plans.json ~/.cache/planalyze/logs.json