Switch to html5-compatible fragment encoding for wikipedia URLs

Fixes #3269

In the HTML4 days, fragments weren't allowed to have `%` signs, so
mediawiki generated fragments with `%` replaced with a `.`

In HTML5, fragments can have % encoded characters, and so in 2017
wikipedia switched over to this for their automatically generated
fragments, while keeping the "dot" versions available as a fallback.

However, we have been automatically converting all fragments,
including manually specified anchors that do not have "dot"-encoded
versions available. So we can now simplify everything by just using
the HTML5 percent-encoded fragments.