Reorganize information in CONTRIBUTING.rst, link to ReadTheDocs
Remove google analytics tracking from iframes used in Drupal course about pages

This fixes two issues:
 - if the course pointed to doesn't exist, GA was added to the iframe twice
 - more importantly, we don't want to count these as separate views anyway--
   we just care about the about page itself.
 - I also added a note about where we would put our segment.io widget if we want it
   later

LMS-2581
wordsmithing
fixes firefox image hidden overflow bug.
puts GA id in quotes
Merge pull request #3564 from edx/adam-fix-GA-quotes

puts GA id in quotes
Merge pull request #3551 from edx/victor/remove-iframe-ga

Remove google analytics tracking from iframes used in Drupal course abou...
When sending bulk email check if course is authorized.
Also the idash_mode property is stored seperately for each course.

LMS-2565
Update IFrame documentation
Merge pull request #3527 from edx/sylvia/docs/DOC-295

Add IFrame template documentation
Parameter from_email to send_mail() should be string and not string in list.

LMS-2518
Fixed link to docs.edx.org in readme
Merge pull request #3574 from edx/release

Release
Merge pull request #3562 from caesar2164/giulio/zoom-image-firefox-bugfix

Fixes firefox image hidden overflow bug.
Update drag and drop documentation
Merge pull request #3559 from edx/sylvia/docs/DOC-99

Update drag and drop documentation
Merge pull request #3417 from edx/usman/lms2518-refund-emails

Parameter from_email to send_mail() should be string
Merge pull request #3575 from edx/markhoeber/documentation/fix-link-in-readme

Fixed link to docs.edx.org in readme
Remove email field from CS client user object

This requires cs_comments_service commit 1d50176.
Do not show "View in Studio" links for child verticals of verticals.

STUD-1558