Uncomment thumbnail tests
Decorate test functions that require Pillow JPEG support
actually look for an image to verify that a thumbnail has been generated
Update pep8 to 1.5.7
Fixing pep8 issues
Ignore "deprecated form of raising exception" PEP8 warning

We do this intentionally, to preserve tracebacks
Address pylint issue from diff-cover
Implements keyword sub feature for bulk emails

This commit pulls in changes from #4487 that implements keyword
substitution for bulk emails. With these changes, an instructor can
include keywords in their bulk emails which will be automatically substituted
with the corresponding value for the recepient of the email. Keywords are
of the form %%keyword%%, and the keywords implemented in this commit include:

%%USER_ID%% => anonymous_user_id
%%USER_FULLNAME%% => user profile name
%%COURSE_DISPLAY_NAME%% => display name of the course
%%COURSE_END_DATE%% => end date of the course

Client-side validations have also been implemented to ensure that only emails
with well-formed keywords can be sent.
The architecture is designed such that adding in new keywords in the future
would be relatively straight-forward.
Add comments about disabled pep8 rules
Move simplefilter comments before call
For methods on PasswordHistory, only calculate if necessary

Check settings.FEATURES["ADVANCED_SECURITY"] first
Change openedx footer image to a non-broken link
fix enrollment issue in instructor dash (ECOM-776)
fixes failing unit test in course_groups
Fix typo in cms unit test
Re-add and clarify comment
fix git_export tests
Make a generic django field for storing OpaqueKey classes
Extract common base functionality for storing xblock fields in the relation db
Move lms.lib.xblock into lms.djangoapps.lms_xblock in preparation add database backed configuration to it