rebased and resolve conficts with cdoge/registration_codes

feature enhancement request: added transaction group name text field to the download buttons as an extra optional query paramerter

This is not currently being used in production and will not
be used moving forward since logs are remoted using splunk

STUD-1725

Move modulestore django settings migration code down into the modulestore layer so it works for all django-based callers.

Disabled by default.

Disabled by default.

Refactor get_parent_locations STUD-1663

Added ORA2_FILE_PREFIX setting

refactor _generate_items_for_subtask not to chunk queries

use django's iterator methor for querysets

remove bulk email query settings

use read_replica if available, since this is read-only

update changelog

Support incremental conversion of events from the old API to the new, in order to ensure the new system is working, enrollment events have been modified to make use of the new API.

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

Changed "Status Page" -> "Page".

UX:
    support for displaying built-in tabs
    restored drag and drop on Studio Pages
    additional styling for fixed state on Studio Pages
    add a new page action added to bottom of Studio Pages

Dev
    changes for viewing tabs in studio,
    refactored the tab code,
    decoupled the code from django layer.
    is_hideable flag on tabs
    get_discussion method is needed to continue to support
external_discussion links for now since used by 6.00x course.
    override the __eq__ operator to support comparing with
dict-type tabs.

Test
    moved test code to common,
    added acceptance test for built-in pages
    added additional unit tests for tabs.
    changed test_split_modulestore test to support serializing objects
that are fields in a Course.

Env:
    updated environment configuration settings so they are
    consistent for both cms and lms.

third_party_auth contains a working settings mechanism, the start of the provider interface + 3 implementations (Google, Mozilla Persona, LinkedIn), and a stub for the auth pipeline. Modified existing lms settings files to use but deactivate the module.

To mark a language as dark-launched, add it to the DARK_LANGUAGES
django conf setting. To activate a dark-launched language, set he
query parameter `preview-lang` to the language code on any url.

[LMS-2045]
[LMS-2077]
[LMS-2076]

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.