Disable "protocol" check from html_lint.py

This actively dissuades from using HTTPS URLs, favoring
protocol-relative ones. Even ignoring HTTPS-only URLs as a best
practice, given most onion services are hosted as HTTP sites,
they'd become HTTP links instead of HTTPS.

So let's just suppress this rule and link to the correct protocol.

Makefile

@@ -82,7 +82,7 @@ fix: ruff ## Apply automatic fixes.
 .PHONY: html-lint
 html-lint:  ## Validate HTML in web application template files.
 	@echo "███ Linting application templates..."
-	@html_lint.py --printfilename --disable=optional_tag,extra_whitespace,indentation,names,quotation \
+	@html_lint.py --printfilename --disable=optional_tag,extra_whitespace,indentation,names,quotation,protocol \
 		securedrop/source_templates/*.html securedrop/journalist_templates/*.html
 	@echo