Logistration: Include course ID in analytics events for login/registration

[ECOM-625](https://openedx.atlassian.net/browse/ECOM-625): Segment authentication and registration events don't record the course id in the label property

Changes:
* The client now sends an `analytics` parameter to the server, containing additional info to be included in analytics events.  The login/registration models populate this with the value of the `?course_id` querystring param, if one is provided.

* I also fixed an indentation issue in the JS test specs, so this diff looks bigger than it actually is.  You can view the diff without these whitespace changes here: https://github.com/edx/edx-platform/compare/will/combine-reg-login-form...will/logistration-include-course-in-analytics?expand=1&w=1

Reviewers: @AlasdairSwan @rlucioni 
I think this comment should be updated to read something like:
```
You can optionally send an `analytics` param with a JSON-encoded
object with additional info to include in the registration analytics event.
Currently, the only supported field is "enroll_course_id" to indicate
that the user registered while enrolling in a particular course.
```