Fixes to random letter generation:

* If more than 26 letters requested, don't get into an infinite loop.
* Don't give away repeat letters: always give word length + num random
    letters even if that means actually adding more random letters than
    requested due to repeat letters in the word.

www/bagels/bagels.js

@@ -60,12 +60,12 @@ class Bagels {
             if (iters++ > maxIters) return null;
         } while (!regExp.test(word) || !isValidWord(word));
 
-        let numLetters = settings.num_random_letters;
+        let numLetters = Math.min(26, word.length + settings.num_random_letters);
         let availableLetters;
         let availableLettersValid = false;
         do {
             availableLetters = new Set(word.split(''));
-            for (let i = 0; i < numLetters; i++) {
+            for (let i = availableLetters.size; i < numLetters; i++) {
                 let newLetter;
                 do {
                     newLetter = getRandomLetter();