refactor: favor directly modifying DOM attributes over setAttribute

According to https://stackoverflow.com/a/57633533 setAttribute()
invokes the HTML parser, which can trigger a CSP violation.
This change refactors style element construction/manipulation to not leverage setAttribute().

src/js/ZoomPane.js

@@ -242,7 +242,7 @@ export default class ZoomPane {
     removeClasses(this.el, this.closingClasses);
     removeClasses(this.el, this.inlineClasses);
 
-    this.el.setAttribute("style", "");
+    this.el.style = "";
 
     // The window could have been resized above or below `inline`
     // limits since the ZoomPane was shown. Because of this, we

src/js/injectBaseStylesheet.js

@@ -57,10 +57,11 @@ export default function injectBaseStylesheet() {
 
   const styleEl = document.createElement("style");
   styleEl.type = "text/css";
-  styleEl.classList.add("drift-base-styles");
-
-  styleEl.appendChild(document.createTextNode(RULES));
+  styleEl.classList = "drift-base-styles";
 
+  styleEl.textContent = RULES;
   const head = document.head;
-  head.insertBefore(styleEl, head.firstChild);
+
+  const allHeadElements = head.getElementsByTagName("*");
+  allHeadElements.innerHTML = styleEl + allHeadElements.innerHTML;
 }