Set default image width/alignment with a sphinx extension

[abidingabi]

Previously, this value was set with css. This worked fine for the HTML build, but did nothing for the LaTeX build. This way, the default width applies to all build formats.

[abidingabi]

As an example of the PDF output that is produced, see https://github.com/abidingabi/gm0/suites/17542735304/artifacts/1002238368.

[abidingabi]

I spotted a bug: tags.has("html") in conf.py does not work because

Only tags set via the -t command-line option or via tags.add('tag') can be queried using tags.has('tag'). Note that the current builder tag is not available in conf.py, as it is created after the builder is initialized. — https://www.sphinx-doc.org/en/master/usage/configuration.html

[abidingabi]

PDF built from this branch: https://github.com/gamemanual0/gm0/suites/17544428062/artifacts/1002339029.

Before:

After:

[abidingabi]

Before the above change:

After: