mermaid: support pan & zoom for graphs

[McShelby]

Mermaid generates a width=100% for certain diagram types - e.g the gantt diagram. On a narrow website this may cause a busy graph to be hard to read.

The mermaid docs expose this behaviour, too, as seen here.

Also related to this issue: If mermaid does not generate a width=100% the diagramm may overflow out of view if the browser window is too narrow - without generating scroll bars. This is clearly an issue of our CSS and also affects print output.

Nevertheless I don't have an idea how to fix this in a satisfying way for all users.

This is a copy of matcornic/hugo-theme-learn#257

[McShelby]

The good news is, that since upgrading our mermaid version in #7, every mermaid graph has a width=100%.

This leaves the problem how to handle big graphs on small screens.

Some kind of zoom mode?