Shift enroll/unenroll logic to CourseEnrollment model, add is_active and mode.

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.
Add deserialize logic for metadata fields.
STUD-640

Handle the case of the double-quotes appearing within the string.

We don't need to worry about the whole concatenated ID being
double-quoted, since the concatenation was done by overriding xml_module's
export method.

Use the method already fined in XmlDescriptor.