http: document source of bootstrap/bootswatch theme

Source is
git+https://github.com/twbs/bootstrap.git@a78dc3aed640a35914361b837ce24573a0515e19#yeti/bootstrap.min.css
but our version has minor modifications:
- removed link to font file
- minor font and line height adjustments
- added SPDX header

Diffing minimized CSS file is a mess so use something like
$ awk '{gsub(/{|}|;/,"&\n"); print}'
to get something more legible.