webapp/assets: Use showdown-highlight from npm instead of vendoring it
showdown-highlight is a plugin for showdownjs enabling to highlight a code block with highlightjs when rendering markdown to html.
There was some issues at the time to use the npm version but it is no longer the case so fetch that code from the npm registry instead of vendoring it in swh-web frontend code.
That updated version also fixes some rendering issues related to code blocks with no explicitly declared programming language (for instance the last rendered code block in the README of the https://github.com/antoine-de/structopt origin is now correctly rendered after these changes).