Implement a BaseStudentModuleHistory

This abstract class contains most of the fields (aside from the id and
foreign key to StudentModule that the subclasses need to manage).  It
also provides a get_history method that abstracts searching across
multiple backends.

Move router code to openedx/core
We need to use it from cms and lms.
Ensure aws_migrate can be used for migrating both the lms and cms.

Handle queries directed to student_module_history vs default and the
extra queries generated by Django 1.8 (SAVEPOINTS, etc).

Additionally, flag testing classes as multi_db so that Django will
flush the non-default database between unit tests.

Further decouple the foreignkey relation between csm and csmhe

When calling StudentModule().delete() Django will try to delete CSMHE
objects, but naively does so in the database, not by consulting the
database router.

Instead, we disable django cascading deletes and listen for post_delete
signals and clean up CSMHE by hand.

Add feature flags for CSMHE
One to turn it on/off so we can control the deploy.
The other will control whether or not we read from two database tables
or one when searching.

Update tests to explicitly use this get_history method rather than
looking directly into StudentModuleHistory or
StudentModuleHistoryExtended.

Inform lettuce to avoid the coursewarehistoryextended app

Otherwise it fails when it can't find features/ in that app.

Add Pg support, this is not tested automatically.

lms/djangoapps/courseware/tests/test_submitting_problems.py

@@ -19,7 +19,7 @@ from capa.tests.response_xml_factory import (
     CodeResponseXMLFactory,
 )
 from courseware import grades
-from courseware.models import StudentModule, StudentModuleHistory
+from courseware.models import StudentModule, BaseStudentModuleHistory
 from courseware.tests.helpers import LoginEnrollmentTestCase
 from lms.djangoapps.lms_xblock.runtime import quote_slashes
 from student.tests.factories import UserFactory
@@ -121,6 +121,8 @@ class TestSubmittingProblems(ModuleStoreTestCase, LoginEnrollmentTestCase, Probl
     Check that a course gets graded properly.
     """
 
+    # Tell Django to clean out all databases, not just default
+    multi_db = True
     # arbitrary constant
     COURSE_SLUG = "100"
     COURSE_NAME = "test_course"
@@ -319,6 +321,9 @@ class TestCourseGrader(TestSubmittingProblems):
     """
     Suite of tests for the course grader.
     """
+    # Tell Django to clean out all databases, not just default
+    multi_db = True
+
     def basic_setup(self, late=False, reset=False, showanswer=False):
         """
         Set up a simple course for testing basic grading functionality.
@@ -451,26 +456,20 @@ class TestCourseGrader(TestSubmittingProblems):
         self.submit_question_answer('p1', {'2_1': u'Correct'})
 
         # Now fetch the state entry for that problem.
-        student_module = StudentModule.objects.get(
+        student_module = StudentModule.objects.filter(
             course_id=self.course.id,
             student=self.student_user
         )
         # count how many state history entries there are
-        baseline = StudentModuleHistory.objects.filter(
-            student_module=student_module
-        )
-        baseline_count = baseline.count()
-        self.assertEqual(baseline_count, 3)
+        baseline = BaseStudentModuleHistory.get_history(student_module)
+        self.assertEqual(len(baseline), 3)
 
         # now click "show answer"
         self.show_question_answer('p1')
 
         # check that we don't have more state history entries
-        csmh = StudentModuleHistory.objects.filter(
-            student_module=student_module
-        )
-        current_count = csmh.count()
-        self.assertEqual(current_count, 3)
+        csmh = BaseStudentModuleHistory.get_history(student_module)
+        self.assertEqual(len(csmh), 3)
 
     def test_grade_with_max_score_cache(self):
         """
@@ -713,6 +712,8 @@ class TestCourseGrader(TestSubmittingProblems):
 @attr('shard_1')
 class ProblemWithUploadedFilesTest(TestSubmittingProblems):
     """Tests of problems with uploaded files."""
+    # Tell Django to clean out all databases, not just default
+    multi_db = True
 
     def setUp(self):
         super(ProblemWithUploadedFilesTest, self).setUp()
@@ -768,6 +769,8 @@ class TestPythonGradedResponse(TestSubmittingProblems):
     """
     Check that we can submit a schematic and custom response, and it answers properly.
     """
+    # Tell Django to clean out all databases, not just default
+    multi_db = True
 
     SCHEMATIC_SCRIPT = dedent("""
         # for a schematic response, submission[i] is the json representation