DEV: Extensively use includes() (#17541)

Also, the change in insert-hyperlink (from `this.linkUrl.indexOf("http") === -1` to `!this.linkUrl.startsWith("http")`) was intentional fix: we don't want to prevent users from looking up topics with http in their titles.

app/assets/javascripts/admin/addon/components/admin-theme-editor.js

@@ -41,7 +41,7 @@ export default Component.extend({
     if (["color_definitions"].includes(fieldName)) {
       return "scss";
     }
-    return fieldName && fieldName.indexOf("scss") > -1 ? "scss" : "html";
+    return fieldName && fieldName.includes("scss") ? "scss" : "html";
   },
 
   @discourseComputed("currentTargetName", "fieldName")