Refactor preview_images to use thread instead of subprocess

[oscarlevin]

This fixes the issue of interactive preview images being unable to be generated on Windows.

Instead of starting http.server in a subprocess, this now starts it in a thread. GET messages from the server are sent immediately to log.debug.

Also fixed a trailing semicolon and a unneeded use of os.path.join when creating the URL.

[oscarlevin]

I should say, it would be good to also test this on linux/mac in addition to the testing I've done on Windows.

[rbeezer]

Thanks, @oscarlevin! I will test on Linux here shortly. Perhaps @Alex-Jordan could do a test on a Mac?

[rbeezer]

OK, testing on Linux with sample article. Short answer: looks good.

Messages all look informative and rational (at full verbosity). Correct files generated, and visually look good. File sizes look to be within epsilon of "old" versions in repository.

I'm going to ping @Alex-Jordan off-list.

[rbeezer]

Forgot the caveat. Using pretext/pretext script, and I did not check how current (old?) my Playwright is.

Which prompts me to create: #2168

[oscarlevin]

Accidentally pushed and accidental merge of Master, so force-pushed a reversion.

[rbeezer]

Merged without any Mac testing. Fingers crossed.

Removed extraneous whitespace. Isolated unrelated small fixes from the real changes.

Thanks for backing up our cross-platform guarantees.