Refactor get_parent_locations STUD-1663

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.

Due to issues with require.js and the latest version of the toolbar.
Also included a note in the requirements file explaining the situation, and what
needs to happen before this can be upgrade properly.

Conflicts:
	lms/envs/dev.py

Due to issues with require.js and the latest version of the toolbar.
Also included a note in the requirements file explaining the situation, and what
needs to happen before this can be upgrade properly.

add trailing comma, restore uncommented state of Mongo panel

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]

(STUD-1201)

Revert "Specific django-toolbar version (stable released) and update the panels in dev envs"

This reverts commit a465b082.

Revert "Updated settings for devstack django debug toolbar"

This reverts commit 30199e8a.

This reverts commit a517dc20.

This reverts commit ec01253e, reversing
changes made to ae8c9710.

@jzoldak showed me that setting `USE_I18N = DEBUG` didn't actually accomplish
what I had hoped it would -- changing DEBUG didn't also change USE_I18N.
This pull request accomplishes what I was trying to accomplish, without being
quite so clever about it.

as a first step to separating db connection from modeling
and to support location mapper and contentstore using same db config as modulestore

use to instantiate loc_mapper
test loc_mapper injection into split modulestore

add a few debug messages to get localhost debug information. Also partition out the MongoDjangoToolbar configuration into a separate env config. This is because doing imports on large courses grinds localdev to a halt due to all the stack trace generation.

not common.  This ensures that we load the MITX_FEATURES
token BEFORE deciding to disable CSRF.

A new modulestore backed by mongo that changes the data format to
facilitate easy versioning, sharing content between courses, and fast
lookup of course structure and Scope.settings data.

Conflicts:
	cms/djangoapps/contentstore/tests/test_contentstore.py

add a new cms ENV configuration to use a slow upload file handler to help test progress bars in uploads

DraftModuleStore was originally designed as a mixin, but never used that
way, and with the upcoming changes to use the versioned module store,
never will be. This changes removes a circular dependency between
mongo.py and draft.py.