* run python modernize
* run isort
* fix pylint issues

LEARNER-5123 | Cache VisibleBlocks per course AND user, only fetch the ones the user needs for subsection scoring.

EDUCATOR-1405
EDUCATOR-1451

commit 12880fa21eac0c2e69b2d832d42829f00484561d
Author: Tyler Hallada <thallada@edx.org>
Date:   Fri Aug 11 11:17:22 2017 -0400

    Tweak exam override messaging in progress.html

commit 6f90352dc27b3a199eea7efaf84b72df50f570a9
Merge: e0e9f83789 7c535f5f
Author: Tyler Hallada <thallada@edx.org>
Date:   Thu Aug 10 17:38:13 2017 -0400

    Merge remote-tracking branch 'origin/master' into EDUCATOR-926

commit e0e9f83789efa837577cedbcd5807c6fdba249e4
Author: Tyler Hallada <thallada@edx.org>
Date:   Thu Aug 10 17:35:26 2017 -0400

    Revert "Comment out actual override and log instead"

    This reverts commit 4953cf30.

commit 27d6b537c8915ecd1af8af51c8f9509db6ea0a26
Author: Tyler Hallada <thallada@edx.org>
Date:   Thu Aug 10 17:34:56 2017 -0400

    Revert "Fix tests"

    This reverts commit 1b2fec21.

commit cabddd504ddcb4f92aeed8b5fec4bff0d64a8de3
Author: Tyler Hallada <thallada@edx.org>
Date:   Thu Aug 10 17:34:36 2017 -0400

    Revert "Remove override behavior and log instead"

    This reverts commit 1ab18803.

commit dc9407f4606804e00d5aee4e80d309c7d7f9c437
Author: Tyler Hallada <thallada@edx.org>
Date:   Thu Aug 10 17:34:15 2017 -0400

    Revert "Fix python tests and use proctoring 1.0.0"

    This reverts commit 842ce836.

Also add it to the course home page.

incoming scores.

TNL-6697

These checks are causing SQL query numbers to scale linearly with the
number of subsections being created/updated, and the errors they
check for have not been seen in prod.

TNL-6225

Some old mongo courses do not have this field, and the team has opted
to allow null values rather than inserting a default. This change
affects both course and subsection grades.

TNL-6408

for TNL-5045

TNL-5930

for TNL-5895

The column is populated with the current timestamp when a passing grade
is persisted for the first time.  "Passing grade" is defined as having a
non-blank value for letter_grade.

TNL-5888

This reverts commit 9aa35cd8.

Includes unit tests
For TNL-5310

This reverts commit 0ea8e1b9, reversing
changes made to e3db08f1.

Potential Grades revert PR

This reverts commit 663fbbba, reversing
changes made to 3f7441c8.

TNL-5599

TNL-5600

TNL-5458
TNL-5493

TNL-5475

We already have logic to serialize BUL objects to string on save,
but the corresponding deserialization logic was missing.

Small changes to the grades djangoapp
    -sets up mock_get_score context handler
    -fixes an issue with not invalidating lazy scores property
    -code quality fixes
    -query count optimization in PersistentSubscetionGrade.create
    -adds atomic blocks to avoid IntegrityErrors corrupting an entire request

Hooks the pre-existing grades code into our new PersistentSubsectionGrade data
model. Includes test updates, and some minor changes to the data model that
were discovered in testing.