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 white spaces inside navigation
Change the default dir for loading xml files.
This should always be overridden by the testcases if needed.
Do not create a modulestore unnecessarily in tests
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
Merge pull request #5731 from Stanford-Online/ataki/upstream

Limit Upload File Sizes to GridFS
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.
WL-105 Show course cards sorted by start dates, oldest first.
Merge pull request #6198 from edx/afzaledx/WL-105

WL-105 Show course cards sorted by start dates, oldest first.
Merge pull request #6211 from Shrhawk/shr/bug/TNL-964-navbar-display-display-issue

Fix white spaces inside navigation
Don't break up i18n string
Double quotes for i18n string
Add comments about disabled pep8 rules
Move simplefilter comments before call
Merge pull request #6111 from edx/zoldak/modulestore-test-config

Unit test speed improvements
Replace &mdash; with literal emdash
Merge pull request #6225 from edx/studio-i18n-improvements

Don't break up i18n string