Overhaul the version history / comparison

[dtdesign]

The comparison currently is a plain view with the raw HTML on both sides. Even the previous variant with bbcode was a subpar solution because it would report all kinds of whitespace changes and other meaningless adjustments.

We should see if we can figure out a way to provide a meaningful comparison using the rendered HTML rather than the plain view. Possibly convert <woltlab-metacode> into the bbcode representation to avoid loading data and to better handle removed content, for example, deleted attachments that could not be rendered at all.