Revert "Update Enrollment failed message with status code"

This reverts commit 357a2a48.

Allow site column to be nullable, with no default. Related to #968.

ECOM-6230

[SOL-2109] Change the default values of max_uses field in coupon form.

When the user selects 'Can be used multiple times by multiple users'
usage limitation option, the default value would be 1 which doesn't
make sense for multi-use vouchers. Instead now it's default is blank
which, if left that way, will default to 10000 which is the arbitrary
limit that Oscar sets for this attribute, and the minimum limit is set
to 2. Values of other usage limitations are left the same.

https://openedx.atlassian.net/browse/SOL-2109

Missing migrations

[SOL-1953] New Receipt Page Backend

Added Custom Code, Client, Category fields on coupon dashboard

- Using the latest version of Firefox on Travis
- Requiring Selenium 3.0.1 or later
- Added geckodriver to connect Selenium to Firefox

I have removed the quiet argument so that we can see exactly what is installed when this command is called. This is primarily useful for knowing what is installed by Travis.

Add UTM tracking for order referrals

ECOM-5539

Updated i18n-tool version, .travis.yml and Makefile files changed

[SOL-2042] Change the email validation in offer models to disregard sub domains.

If a domain is set as a valid email domain then any emails that match that domain
are accepted, including ones with different sub domains.
If a domain with sub domain is set as valid then only emails with that sub domain
are accepted.

This fixes the invalid call to BasketAttribute.objects.update_or_create() that has caused errors for users attempting to modify an existing basket.

ECOM-5955

Making a single call to retrieve the value, rather than attempting to create it (which is done via migration).

ECOM-5955

Corrected test names and docstrings

ECOM-5955

Update Enrollment failed message with status code

Change edx-django-release-util version.

[SOL-2083] Check if user is enrolled in course only if the request arrives from the Offer Landing Page

[SOL-2083] Check if user is enrolled in course only if the request arrives from the Offer Landing Page

Remove unnecessary request data preparation

Disallow showing the note about verified certificate in baskets with credit seats.

Add email validation for coupon form.

Refactor create coupon

Change the logic for when to display an empty offer error.

Display error message when no offers exist for a coupon.