Encode Mermaid Images

[rowanc1]

It is quite easy to convert images of mermaid graphs using the https://mermaid.ink/ service.

I think this should be a stopgap use until we figure out our chromium snapshots. Can download the image similar to how we would for any other remote image (with some up-front base64 encoding).

This would help with tex, word and pdf outputs with these diagrams, but does have a network request, which we want to continue to avoid (e.g. for @jkmacc-LANL and others on more restricted networks).

[rowanc1]

This is picked up in #2132