Fix subscripts in SVG

[radiantBear]

SVG <text> elements do not support HTML <sub> elements in their content. Vamonos.resolveSubscript uses <sub> tags to render subscripts, which works great for subscripts rendered in HTML (such as G_f in the following screenshot), but does not work for subscripts rendered in SVG (such as t_f in the following screenshot). This means that graphs currently render with the <sub> tags printing in plaintext:

Instead of using <sub> elements in SVG, <tspan> can be used to style a subset of text within an SVG text block. Vamonos.resolveSvgSubscript uses this to style subscripts for SVG. These subscripts are styled:

• With baseline-shift='sub' to render below the normal text line in the SVG way
• With font-size='75%' to match Bootstrap's size difference for subscripts (applied to <sub> tags in HTML)

This fixes the subscripts in the graphs without breaking subscripts rendered in HTML:

Important

This also changes the graph's vertex labels to render as HTML instead of escaped text (escaping text was a second problem making the <sub> tags visible in the graph). Labels are already rendered as HTML outside the graph so that the <sub> tags work properly, so this seems like a reasonable choice here as well. However, if a user were to create a vertex label like "<a>", this would render as a (broken) anchor tag instead of the literal string "<a>".