Add a basic tooltip showing who reacted

This adds a first attempt at tooltip showing who reacted to a message. It
doesn't limit senders or position the tooltip nicely, but the info is there at
least.

Part of https://github.com/vector-im/riot-web/issues/9722

src/HtmlUtils.js

@@ -107,6 +107,17 @@ function unicodeToImage(str, addAlt) {
     return str;
 }
 
+/**
+ * Returns the shortcode for an emoji character.
+ *
+ * @param {String} char The emoji character
+ * @return {String} The shortcode (such as :thumbup:)
+ */
+export function unicodeToShort(char) {
+    const unicode = emojione.jsEscapeMap[char];
+    return emojione.mapUnicodeToShort()[unicode];
+}
+
 /**
  * Given one or more unicode characters (represented by unicode
  * character number), return an image node with the corresponding