Rework mermaid integration to support off-DOM rendering

[knolleary]

This is a rework of how we integrate mermaid into the editor.

As reported in #4323, if we try to render a mermaid diagram in an element that isn't currently visible, it results in an error. This is an "expected" behaviour of using mermaid.init/run.

In this PR, I've updated to the latest mermaid and modified how we drive it. We now do the work to call mermaid.render with the diagram text to get the svg back and insert it ourselves.

The downside is we have to do the error handling ourselves - so instead of the mermaid 'bomb' image, it now displays the error in text. It isn't as pretty, but to be honest, it feels more useful to know what the error was rather than just the fact there was an error.

I've also updated the internal APIs so nothing should be calling the global mermaid directly - we take care of that.

Whilst I was at it, I also fixed #4358 as it the help sidebar wasn't making the call to render mermaid.