Update pillow to 2.6.1
Limit Upload File Sizes to GridFS.

This commit puts a limit on the size of files that
course staff can upload to MongoDB. The limit is
enforced on the frontend in javascript as well
as backend via the /upload endpoint.

The limit is hard-coded in cms/envs/common.py and
may be changed according to the user's custom
needs.

If the user tries to upload a file that's too
large, an error message will pop up, with a
customizable url that pointing the user to an
external page with an alternate upload procedure.
This url is specified im cms/envs/common.py.
If not set, this url will not be displayed.
Fix Mako templates to use latest request context

Fixed the edX middleware to lazily create request
context for Mako templates, so that it is based on
any changes made to the request object.

Verified that with this fix the "Instructor" tab is
correctly hidden when a staff member is viewing
the course as a student.
Fix white spaces inside navigation
Support cohorting students via a CSV File.

TNL-735
add find_fixme task
Updating testing.md for find_fixme
Merge pull request #5961 from edx/dan-f/cohort-csv-upload

Upload CSV file for cohorting students
Merge pull request #6173 from edx/clytwynec/add_find_fixme_method

add find_fixme task
Merge pull request #6189 from edx/andya/fix-masquerade

Fix Mako templates to always use updated request context
Add DEFAULT_FILE_STORAGE configuration.

Tests in common are run with both CMS and LMS configuration (with different env files). Ensure the DEFAULT_FILE_STORAGE variables are in both environments.
Return the filename as returned from default storage's save.

It might be different from the requested filename.
Merge pull request #6215 from edx/christina/test-file-cleanup

Add DEFAULT_FILE_STORAGE configuration.
Adding yaml_config, a new settings module

This reads configuration directly from yaml
and imports it directly into settings
Merge pull request #6188 from edx/jarv/config-refactor-wip

moving defaults for yaml config out of common.py
round-off weight, min_count and drop_count values while returing grading policy
TNL-884
Merge pull request #6092 from edx/zub/bugfix/tnl-884-save-weight-in-gradingpolicy

round-off weight value while returing grading policy
MA-182 Fix Mobile Subtitles.
Merge pull request #6221 from edx/hotfix/MA-182

MA-182 Fix Mobile Subtitles.
Merge pull request #4276 from edx/db/update-pillow

Update pillow to 2.6.1