Revert "SUST-35 Implementation of course_key_from_string_or_404 to return course_key or raise a 404"

Revert "SUST-35 Implementation of course_key_from_string_or_404 to return course_key or raise a 404"

-Updated method "course_key_from_string_or_404" to raise message too.
-Wrote tests for "course_key_from_string_or_404" when exception message is given.
-Modified existing methods to use "ddt.data".
-Used Splunk logs to find exectly where we were getting "Invalid Key Error"
-Updated Views where we were getting "Invalid Key Error" in splunk logs.
-Wrote tests for those View End points where we were getting "Invalid Key Error"

Change name of method to clear up confusion with the access.has_access method.

This commit adds all of cms.

These keys are now objects with a limited interface, and the particular
internal representation is managed by the data storage layer (the
modulestore).

For the LMS, there should be no outward-facing changes to the system.
The keys are, for now, a change to internal representation only. For
Studio, the new serialized form of the keys is used in urls, to allow
for further migration in the future.

Co-Author: Andy Armstrong <andya@edx.org>
Co-Author: Christina Roberts <christina@edx.org>
Co-Author: David Baumgold <db@edx.org>
Co-Author: Diana Huang <dkh@edx.org>
Co-Author: Don Mitchell <dmitchell@edx.org>
Co-Author: Julia Hansbrough <julia@edx.org>
Co-Author: Nimisha Asthagiri <nasthagiri@edx.org>
Co-Author: Sarina Canelake <sarina@edx.org>

[LMS-2370]

STUD-1006

Ensure user admin screen gets the union of all possibly matching group names.
Smarter default group naming.
STUD-1003

Move RESTful URLs into our "done" section.

Features coming down the pipe will want to be able to:
* Refer to enrollments before they are actually activated (approval step).
* See what courses a user used to be enrolled in for when they re-enroll in
  the same course, or a different run of that course.
* Have different "modes" of enrolling in a course, representing things like
  honor certificate enrollment, auditing (no certs), etc.

This change adds an is_active flag and mode (with default being "honor").
The commit is only as large as it is because many parts of the codebase were
manipulating enrollments by adding and removing CourseEnrollment objects
directly. It was necessary to create classmethods on CourseEnrollment to
encapsulate this functionality and then port everything over to using them.

The migration to add columns has been tested on a prod replica, and seems to be
fine for running on a live system with single digit millions of rows of
enrollments.

Remove _

Minor cleanup.

STUD-554

Code review feedback.