[CHANGE] Mermaid images

[Rob Beezer]

There have been some changes to how Mermaid images are handled. Read on if you
are using this feature.

* Filenames for images used in static formats (like LaTeX/PDF, or EPUB) are now
derived from a @label attribute on the #mermaid element. No longer from an
@xml:id on an enclosing #image. This is part of a broader change like this a
while back that applied to other images described by a source language. The
change is completely backward-compatible, so there is no rush to make changes.

* We no longer make a color version and a B&W version for each such image. So
filenames have changed (they no longer need a suffix). The latest version of
the CLI (last night, thanks Oscar!) incorporates this change so everything
should just work. If not, force a regeneration of assets. And if that doesn't
work, report on -support, please.

* The only missing feature is that if you set your publisher file to create a
print version of LaTeX, then you would have gotten the B&W versions of the
images in your PDF. Now, you can accomplish this (for any format) via the
publisher file by electing the "neutral" Mermaid theme.

* We now have the ability to make SVG versions of these images, which is
necessary for EPUB output. Again, should be automatic with the CLI.

Rob