Disable editor autocompletion for didactic reasons

In our educational environment, autocompletion is generally not
desirable:

* Beginners might be confused by the suggestions.
* If we want students to memoize the keywords, suggesting them is
  counterproductive.

Of course, autocompletion also has its benefits. If necessary, we can
revisit this decision at a later time. For example, we could have
different settings for beginner and complex tasks.

assets/js/editor.js

@@ -861,8 +861,8 @@ class Editor {
             mode: "ace/mode/java",
             newLineMode: "auto",
             tabSize: 4,
-            enableBasicAutocompletion: true,
-            enableLiveAutocompletion: true,
+            enableBasicAutocompletion: false,
+            enableLiveAutocompletion: false,
             maxLines: Infinity,
             fontFamily: "Menlo, Monaco, Consolas, \"Courier New\", monospace",
             fontSize: "10.5pt",