Import/Export Libraries

Import and Export content libraries via XML.
@Kelketek upstream reviewers asked to remove this entire "Tools" block from the very first PR, so you'll have to add it. I've saved a gist for you, though: https://gist.github.com/e-kolpakov/afc0942f781c63b4ca42.

Should go at the very bottom of `<div class="wrapper wrapper-l">`, just after closing `</nav>`, unless later PRs introduced other changes.