Rationalize CSS in SVG graph visualizations

[kasbah]

• Mix of .module.css and non-module
• Styles overriding each other .e.g. Stock-flow diagrams with signed links #905 (comment)

If we look at what CSS affects simple stock and flow graphs we get:

1. packages/frontend/src/stdlib/svg_styles.module.css - Stock/transition boxes, places, blue links
2. packages/frontend/src/stdlib/styles.module.css - HTML box borders and padding
3. packages/frontend/src/stdlib/analyses/graph_visualization.css - Container scrolling
4. packages/frontend/src/visualization/graph_svg.css - Graph, node, edge, double-line arrows, arrowhead markers

[epatters]

I've got a fix for the immediate bug in #915 but I'm going to leave this open because the situation remains overly complicated and chaotic.