clean diagrams: ASCII for .txt, SVG for .html

This uses aasvg to convert pure ASCII diagrams into clean line drawings for those formats that can handle them.

While aasvg is in debian, introducing svg also means kramdown wants to run svgcheck to verify that the svg is RFC-conformant.

svgcheck is not in debian yet, so we need to install it via pip for building the editor's copy.

I'm looking at getting svgcheck (and its dependency rfctools-common) into debian proper, but the current releases on PyPI have problems with their built-in test suites:

• https://github.com/ietf-tools/svgcheck/issues/1
• https://github.com/ietf-tools/rfctools-common/issues/1

We're installing the dependencies for svgcheck (and rfctools_common) via apt, but pip3 will install the as yet unpackaged modules.