[site] resurrect sidebar and place on all pages
[site] attempt to put static config somewhere sensible

had wanted to put it in `Config.Site` but can't get `src/_tags` to work to specify `cow.syntax` package for `<src/config.ml.depends>` at least, required as some of the static config is `Html.t` fragments. so it's ended up in `Site_config`
[site] use `Headers` from `Cowabloga`
[courses] start to fill out URL dispatch
[courses] name is now metadata, not URL (index -> all)
[courses] complete list now works; pgt and ugt next
[courses] pgt, ugt, tt and reqs all working again
whitespace
[todo] update
[courses] minor refactoring
[site] support json content-type
[courses] make paths absolute to handle presence/absence trailing / on URI