Don't reparse HTML files on save

For #5279.

Back when we wanted to reload HTML files in the browser on save, it made sense to fully rescan/reparse the document on save, since we were going to serve up a new version of the instrumented HTML. However, now that we no longer reload the HTML file, we don't want to reparse the doc, because that will generate new IDs which won't be reflected in the browser (since the browser will never request the reinstrumented HTML).

@gruehle 

This is a pull request into the sprint-31-hotfix branch since we need to re-roll the official sprint 31 build. We will also merge that branch into master later.