Disable submit button on save click

common/lib/xmodule/xmodule/js/src/capa/display.coffee

@@ -412,11 +412,13 @@ class @Problem
       @save_internal()
 
   save_internal: =>
+    @enableCheckButton false
     Logger.log 'problem_save', @answers
     $.postWithPrefix "#{@url}/problem_save", @answers, (response) =>
       saveMessage = response.msg
       @gentle_alert saveMessage
       @updateProgress response
+      @enableCheckButton true
 
   refreshMath: (event, element) =>
     element = event.target unless element