Move bergamot widget into blog theme

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

assets/js/katex-component.js

@@ -0,0 +1,31 @@
+class KatexExpressionShim extends HTMLElement {
+    static observedAttributes = ["expression", "katex-options"];
+    targetSpan;
+
+    constructor() {
+        super();
+    }
+
+    doRender() {
+        if (!this.targetSpan) return;
+
+        const options = this.hasAttribute("katex-options") ?
+            this.getAttribute("katex-options") : {};
+        katex.render(
+            this.getAttribute("expression"),
+            this.targetSpan,
+            JSON.parse(options)
+        );
+    }
+
+    connectedCallback() {
+        this.targetSpan = document.createElement('span');
+        this.appendChild(this.targetSpan);
+        this.doRender();
+    }
+
+    attributeChangedCallback(name, oldValue, newValue) {
+        this.doRender();
+    }
+}
+customElements.define("katex-expression", KatexExpressionShim);