Massive JavaScript cleanup

- Use JSX for templates
- Docblock/comment everything
- Mostly passes ESLint (still some work to do)
- Lots of renaming, refactoring, etc.

CSS hasn't been updated yet.

js/lib/Translator.js

@@ -0,0 +1,64 @@
+/**
+ * The `Translator` class translates strings using the loaded localization.
+ */
+export default class Translator {
+  constructor() {
+    /**
+     * A map of translation keys to their translated values.
+     *
+     * @type {Object}
+     * @public
+     */
+    this.translations = {};
+  }
+
+  /**
+   * Determine the key of a translation that should be used for the given count.
+   * The default implementation is for English plurals. It should be overridden
+   * by a locale's JavaScript file if necessary.
+   *
+   * @param {Integer} count
+   * @return {String}
+   * @public
+   */
+  plural(count) {
+    return count === 1 ? 'one' : 'other';
+  }
+
+  /**
+   * Translate a string.
+   *
+   * @param {String} key
+   * @param {Object} input
+   * @return {String}
+   */
+  trans(key, input = {}) {
+    const parts = key.split('.');
+    let translation = this.translations;
+
+    // Drill down into the translation tree to find the translation for this
+    // key.
+    parts.forEach(part => {
+      translation = translation && translation[part];
+    });
+
+    // If this translation has multiple options and a 'count' has been provided
+    // in the input, we'll work out which option to choose using the `plural`
+    // method.
+    if (typeof translation === 'object' && typeof input.count !== 'undefined') {
+      translation = translation[this.plural(input.count)];
+    }
+
+    // If we've found the appropriate translation string, then we'll sub in the
+    // input.
+    if (typeof translation === 'string') {
+      for (const i in input) {
+        translation = translation.replace(new RegExp('{' + i + '}', 'gi'), input[i]);
+      }
+
+      return translation;
+    }
+
+    return key;
+  }
+}