Added an expiration_datetime field to CourseModes, intended to eventually replace expiration_date.

Included relevant schema and data migration file.

LMS-1454

Change delete_item to RESTful URL.

Part of STUD-847.

emit enrollment events

 a11y tests for video player controls [BLD-386]

Added tests to video player controls to check new WAI-ARIA functionality. Added global custom Jasmine matcher to helper.coffee

[BLD-386]

Do JS setup immediately when possible

registration-optional: Allow to hide some optional registration fields #os

setting THEME_NAME to None when it's set to an empty string

Add retries when encountering DatabaseError when updating InstructorTask.

If the YouTube API JS fails to load, the rest of the JS should still run

Sarina/enhance user factories

Fixes BLD-472

Removed duplicate code

Enabled course id clash command tests

Allow to control which of the optional fields from the registration page
are being displayed to the user. Added the configuration variable
`REGISTRATION_OPTIONAL_FIELDS` to control which ones. The default value
shows all the fields as currently; to not display some of the fields,
set the configuration variable with only the fields you want to display:

```
REGISTRATION_OPTIONAL_FIELDS = [
    'level_of_education',
    'gender',
    'year_of_birth',
    'mailing_address',
    'goals'
]
```

Flowerhack/feature/refunds for certificates

Avoid CS search endpoint for non-search queries

adds a new option to ungenerated_certs to use http in the callback url

adding create_user script

* Changed text for "Are you sure you want to refund"...
* Addressed a CR comment adding clarity to student view code
* Converted UTC to pytz.utc

Determine which mode was selected more intelligently.