Extend knowledge validation

In trying to add a skeleton of color content I discovered we have secret requirements for knowledge content. We should detect these in CI. So far I've run into:

1. thumbnail.svg appears to require width/height attributes (https://github.com/google/fonts/commit/bbdf7341966d4cb6034cc2a6084c0f86b6c49f18)
1. lessons appear to require a thumbnail (https://github.com/google/fonts/commit/d24e904e813ea92bc4c128c43518b58123308a10)
1. apparently authors have to be defined; Lesson introducing_color_fonts (in module introducing_type) references undefined author "Roderick Sheeter"
1. user error but nice to pre-check, malformed textproto content :) https://github.com/google/fonts/commit/1c8cddafe54b7a362c623f89e271808ce277ab25
1. related urls are extremely restrictive, must match `^https://www.youtube.com/embed/[a-zA-Z0-9_?=-]+$`; https://github.com/google/fonts/pull/5084
1. if one is silly enough to forget an image (https://github.com/google/fonts/pull/5096/files is missing transgender-flag.svg) that is referenced from your markdown the push will fail, notentirely helpfully reporting the error "Disallowed html attribute img.src = images/transgender-flag.svg"