Prevent people from accidentally comitting comments with unescaped %

I wish there was an autocommenter or something on PRs in this repository that would warn you about unescaped usages of the percentage sign %. It's especially common in URLs. It's very hard to figure what's wrong because the website completely swallows up broken comments. They're only included in the total comment count, but there's no sign of them in the output.

Examples of PRs of people who accidentally made this mistake and had to fix it.

- https://github.com/exercism/website-copy/pull/2195
- https://github.com/exercism/website-copy/pull/2133
- https://github.com/exercism/website-copy/pull/2007
- https://github.com/exercism/website-copy/pull/1408
- https://github.com/exercism/website-copy/pull/1144