Reviewed mermaid viewer hack

Reviewed mermaid viewer hack

Modified with the changes:
- Split out files to make use of the public files system, and so it's
  not a massive chunk of head content loaded on all pages.
- Added a little extra size control during load to reduce bouncing.
- Updated code block targeting just to avoid potention editor conflicts.
- Removed some non-existing CSS variable usages.
- Tweaked hack wording/guidance a little.