* INCR-413: Run python-modernize and isort

* INCR-413: Fix for text_type usage

Augment content_type_gating tests to detect the failure case from the original REVMI-34 implementation

* INCR-230 - Run python-modernize and isort on common/lib/xmodule/xmodule, plus a few minor fixes suggested by Jeremy

* INCR-229 ran modernize on common/lib/xmodule/xmodule/[a-g]*.py except for graders.py

* fix a couple xss-commit-linter issues

* incr-172

* incr-177 run modernize

* fixed bug due to running modernize

* INCR-214: python-modernize on common/lib/xmodule/xmodule/modulestore/tests

* Running isort on common/lib/xmodule/xmodule/modulestore/tests

* Resolving issues with mangled comments and fixing int/long test

* INCR-227: python-modernize on common/lib/xmodule/xmodule/video_module, partitions, and util

* pylint will ignore imports from six

* INCR-227 Fixing mangled comments and variable names

* INCR-226 run modernize on common/lib/xmodule/xmodule/modulestore/mongo, split_mongo, and perf_tests

* fix pep8 error

This is part of a larger effort to split packages out into independent,
composable, PyPI installable Python packages.

While this package itself isn't monumental, it represents the bottom of
the dependecy tree and can be split out before any other packages.

* Minimum possible changes were made to merge CapaModule & CapaDescriptor into
  one ProblemBlock class.
* There are no known changes in behavior.
* CapaModule and CapaDescriptor inherited from a number of classes which inherit
  from XModule or XModuleDescriptor but did not depend on them. For all these
  classes the methods were moved to mixins which did not inherit from either and
  then these mixins were added to ProblemBlock in the order which maintains MRO.

The Randomized Content Block XBlock only randomizes the selection of
the children blocks and has unpredictable randomization of
the order of the selected child blocks due to the usage of sets, which
are unordered, for storing the selected blocks. This becomes apparent
when all the available child blocks in a library are chosen for a
Randomized Content Block, to randomize just the order of the child
blocks and not just the selection of the blocks. The order of the
selected blocks ends up being similar for multiple learners.

This change modifies the XBlock to store the selected child blocks in
a list, instead of a set, after randomly shuffling them.

This adds a new django app -- edx-when -- that will copy start and due dates to mysql and allow per-learner overrides in the instructor dashboard, using the existing IDDE interface.
It adds a data migration for existing IDDE data.

* INCR-185 Modernize common/lib/xmodule/xmodule/assetstore and contentstore

* Adding check for python version when making integer contracts

Revert "Revert "REVMI-62 Return gated blocks in the course blocks API response with an authorization denial reason""

Revert "REVMI-62 Return gated blocks in the course blocks API response with an authorization denial reason"

Co-Authored-By: davestgermain <dave@st.germa.in>