If neither start nor advertised_start has been changed from default
don't show the course start date.

This allows us to accept course registrations for courses whose start
date is yet TBD.

All the dialogs in the wiki are now accessible to screen readers.

Also, the "Switch to selected version" button did the odd thing of
reloading the page with the new version selected, but inside a dialog.
That is now fixed.

The merge dialog has a warning that looks kind of funky still, but you
can read it, unlike the previous code.

LMS-1337

AJAX requests on forums pages occasionally fail (usually when a request
to the comments service times out), but the user is not made aware of
the failure. This manifests as either the system not appearing to store
state (e.g. if an attempt to flag a post fails) or something taking
forever to load. Now, a modal will pop up to alert the user that a
request has failed and instruct them to reload the page.

In the longer term, we should fix each user gesture that results in an
AJAX call to gracefully handle a failure by resetting the state of the
world appropriately and aleritng the user.

JIRA: FOR-37

- Doubled up :hover and :focus styling to improve a11y
- Increase contrast of certain UI elements for improved a11y
- Added some image alt text for a11y
- Changed video caption styling to blue and made them underline on hover and added a skip link before video for screenreaders.

Fixes Bugs:
- LMS-1336

With tests, some settings changes
(all should default to not breaking anything for edx)

Added styling for shopping cart User Experience
- Styled shoppingcart list page
- Styled navigation shopping cart button
- Styled receipt page
- Styled course about page for shopping cart courses

Addressed HTML/SCSS issues

Remove offending body class and unnecessary sass changes

Addresses many review comments on stanford shopping cart

* framework for generating order instructions on receipt page
  in shoppingcart.models
* move user_cart_has_item into shoppingcart.models
* move min_course_price_for_currency into course_modes.models
* remove auto activation on purchase
* 2-space indents in templates
* etc

revert indentation on navigation.html for ease of review

pep8 pylint

move logging/error handling from shoppingcart view to model

Addressing @dave changes

LMS/Accounts: Converts many class-based Sass archetype-focused @extends to use Sass placeholder syntax

The close button for the modal dialog was marked up as a paragraph,
containing a graphical text character, inside two div elements, with an
associated click event. This made the button unusable for keyboard
users.

Replaced <divs> with a link element (<a role="button"), adapted the CSS
accordingly, and updated the existing <a> modal close buttons.

Two endpoints are intended to be used by AJAX calls from the web UI with an
authenticated user. When enabled, the value of the preference is an encryption
of the username, to be used as a token for one-click unsubscribe links. The
third endpoint is the target of unsubscribe links, which displays a page with
an appropriate message to the user.

Bourbon's box-shadow mixin has been deprecated, and they recommend to use the
official spec.

The official border-radius mixins were deprecated and removed in Bourbon 3.0.
https://github.com/thoughtbot/bourbon/pull/95

theming - revised variables and link/button styling to account for edX's registration and login constrast/styling changes

lms/mktg - removes margins from social icons and reinstates flex-gutter margin to colophon area of footer