Links that start with a scheme (e.g., `tel:18008675309`) inadvertently
had a URL prefix prepended. Previously, only `mailto:` was handled, but
given the sheer number of [registered URI schemes][uri-schemes], a loose
pattern matcher is used to detect schemes instead.

External links, as identified by the renderer, are now limited to `http`
and `https` schemes.

Fixes #747 and fixes #816.

[uri-schemes]: https://www.iana.org/assignments/uri-schemes/uri-schemes.xhtml