When resuming a question, resolve pre_submit_promises for gaps once t…

…he parent has been submitted.

This fixes a bug where a later part depends on a gap in a gap-fill, but the promise for the gap was never being resolved because it's not submitted on its own.

runtime/scripts/question.js

@@ -992,6 +992,11 @@ Question.prototype = /** @lends Numbas.Question.prototype */
                             resolve();
                         }
                     });
+                    if(part.gaps) {
+                        promise.then(function() {
+                            part.gaps.forEach(function(g) { part_submit_promises[g.path].resolve(); });
+                        });
+                    }
                 }
 
                 q.signals.on('ready',function() {