Add some optional policies around password resets, such as password reuse, forced password resets, allowed frequency of password resets

Add optional feature to lock out accounts after N failed login attempts. Lockouts will last M seconds.

add DB migration and fix earlier mistakes in student migration history

add tests and fix bugs that came out of those unit tests

remove unused import

pep8/pylint

address some PR feedback

fix tests

fix broken test

try to mock time

use freeze-gun to overload the system time to simulate the future

Extend the capabilities of the REGISTRATION_OPTIONAL_FIELDS
configuration variable, to allow to select, for each individual field,
if it should be 'hidden', 'optional' or 'required'.

Rename the configuration variable to REGISTRATION_EXTRA_FIELDS to reflect
the additional capabilities, and updates the defaults.

As extra fields, configurable through the REGISTRATION_EXTRA_FIELDS
variable. Hidden by default.

Tickets: MCKIN-168 MCKIN-184

Note: Studio also has a registration page, which uses the same account
creation page. It should be possible to use it without requiring the
variable from the LMS, as the fields are different.

This does not yet replace the existing per-student anonymous id, but
is intended to do so in the future.

Co-author: Alexander Kryklia <kryklia@edx.org>
Co-author: Ned Batchelder <ned@edx.org>
Co-author: Oleg Marchev <oleg@edx.org>
Co-author: Valera Rozuvan <valera@edx.org>
Co-author: polesye

add migration

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 single enrollment/unenrollment
- Make unenrollment work with multi-email listbox.
- Provide checkbox for auto-enroll on activate if user not yet signed
up.
- Enroll student when activating if auto-enroll flag set.

fix migrations, removing one field and making another nullable.  Update registration page to allow for editing only demographics.

Revert "add migration to remove askbot columns"

This reverts commit 9281f3c1.

Re-add TestCenterUser along with a basic command to make an entry for a given user already in the system.