♻️ Refactor: Move 2 inline event handlers out of line

If MDN explicitly warns user not to use them, we probably shouldn't.

MDN also states: "Note that inline event handlers are blocked as well[...]You should replace them with addEventListener calls[...]" regarding CSP.

assets/js/button-likes.js

@@ -0,0 +1,4 @@
+document.getElementById("button_likes") &&
+  document.getElementById("button_likes").addEventListener("click", () => {
+    process_article();
+  });

assets/js/katex-render.js

@@ -0,0 +1,4 @@
+document.getElementById("katex-render") &&
+  document.getElementById("katex-render").addEventListener("load", () => {
+    renderMathInElement(document.body);
+  });