Improve accessibility for forums post toolbar

Mark toolbar and buttons with appropriate ARIA roles, add them to the
tab order, and make the titles more screen-reader friendly.
Don't try to purchase the same order twice
Catch IOError and ignore if OS X companion file
Remove useless try-except clause
don't remove comments from xml module
Add unicode handling to the signing and verifying logic
edx.org - converts type of log in header to not be all caps
edx.org - adds in aria-describedby attributes for condition-dependent registration input elements
Fix staff-facing error with open ended
edx.org - replaces login/register semantically verbose fieldset and legend elements with simpler markup
add accessibility message if mathjax is detected

use ie conditional html tags to detect for compatible ie versions
LMS: semantically hides the ie-centric browser warning/FF/Chrome suggestion message to avoid confusing/misleading assistive tech-based users.
Merge pull request #1067 from edx/talbs/fix-a11y-browsermessage

LMS: Hide Browser Recommendation Message from Assistive Tech Users
If the current course doesn't have a verified course mode
kick the user out of the verify registration flow.
Merge pull request #1048 from edx/diana/cybersource-unicode-handling

Add unicode handling to the signing and verifying logic
Merge pull request #1040 from edx/diana/no-ordering-twice

Don't try to purchase the same order twice
Merge pull request #1071 from edx/diana/order-integrity-errors

If the current course doesn't have a verified course mode
Mention in studio UI that %%USER_ID%% works
Disabled video autoplay for students
Modified acceptance test to reflect autoplay=false as a default for lms