Added trailing slashes otherwise the course_id regex sometimes eats up get parameters.

LMS-2738
BLD-725: ChoiceGroup i18n.
Extract opaque_keys into a separate library

[LMS-2571]
Fix exception on Internet Explorer when adding first transcript language

Internet Explorer throws an "Invalid argument" exception when uses an empty string as the parameter in ```namedItem``` method.
Change empty string into null value can maintain browser consistency.
Do not persist anonymous ids when exporting for all students of a course.

For large courses this causes export to take a long time and to time out.

LMS-2747
DOC-362, another set of students who don't get email
Two image references incorrectly preceded by a comment
Merge pull request #3910 from edx/ahodges/documentation/images

Two image references incorrectly preceded by a comment
Build 'window.location.origin' for Internet Explorer

As TinyMCE's CodeMirror plug-in uses 'window.location.origin' which is not supported by Internet Explorer, the HTML or raw code editor does not work under Internet Explorer.
We need to create one in order to make these vendor codes work under Internet Explorer.
BLD-1103: Fix ChoiceTextResponse.
Merge pull request #3902 from edx/anton/i18n-choice-group

ChoiceGroup  i18n.
Merge pull request #3879 from edx/usman/lms2747-anonymous-ids-collection

Do not persist anonymous ids when exporting for all students of a course.
Merge pull request #3833 from edx/usman/lms2738-verify-student-urls

Added trailing slashes to verify_student app urls.
JavascriptResponse i18n. [BLD-737]
Merge pull request #3918 from edx/jmclaus/i18n-javascript-response

JavascriptResponse i18n.
Merge pull request #3916 from edx/anton/fix-choice-text-response

Fix ChoiceTextResponse.
Increase async script timeout in lettuce tests
Merge pull request #3892 from louyihua/ie-fix

Fix exception on Internet Explorer when adding first transcript language
Merge pull request #3909 from edx/ahodges/documentation/DOC362

DOC-362, another set of students who don't get email
Increase the default timeout for waiting in lettuce tests