Improve banner semgrep rule

This makes the rule also match something like this:

%section{class: "gl-banner"}

But turns out we don't have anything like this in the codebase anymore, thanks to the last migration day, I guess.

Note: I decided against another rule that would test for HTML-style attributes like %div(class="gl-banner"). We don't have any matches right now and a haml-lint rule that prevents adding anything like that.

Resolves #13 (closed)