Allow a value of 0 for max attempts. STUD-264.

cms/djangoapps/contentstore/features/problem-editor.feature

@@ -52,7 +52,7 @@ Feature: Problem Editor
   Scenario: User cannot type out of range values in an integer number field
     Given I have created a Blank Common Problem
     And I edit and select Settings
-    Then if I set the max attempts to "-3", it displays initially as "-3", and is persisted as "1"
+    Then if I set the max attempts to "-3", it displays initially as "-3", and is persisted as "0"
 
   Scenario: Settings changes are not saved on Cancel
     Given I have created a Blank Common Problem

common/lib/xmodule/xmodule/capa_module.py

@@ -69,7 +69,7 @@ class CapaFields(object):
     max_attempts = StringyInteger(
         display_name="Maximum Attempts",
         help="Defines the number of times a student can try to answer this problem. If the value is not set, infinite attempts are allowed.",
-        values={"min": 1}, scope=Scope.settings
+        values={"min": 0}, scope=Scope.settings
     )
     due = Date(help="Date that this problem is due by", scope=Scope.settings)
     graceperiod = Timedelta(help="Amount of time after the due date that submissions will be accepted", scope=Scope.settings)