Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

When running locally, XBlocks which access third-party software no longer
cause server errors.

Because devstack inherits from aws, the HTTPS flag was implicitly set to 'on'.
This meant that XBlocks attempted to use SSL when dealing with third-party
components like LTI.  Since SSL is not enabled on devstack, this caused server
errors whenever these features ran.

This is primarily to reduce load on MongoDB, where we've lately
had performance problems that we suspect are caused by very
large course structures being evicted from MongoDB's cache. This
may potentially give us a path to better performance as well,
but that's not the goal of this commit.

Surprisingly, LZ4 seemed to actually run more slowly than zlib
for this. Possibly because of some overhead in the Python
bindings? GZip was also surprisingly slow given that it uses
zlib underneath (something like 5x slower).

Use separate cache backend for caching structures.

Abstract out course structure cache.

add datadog metrics for compressed course structure sizes

Since we're using a different cache background, we don't need to have a cache prefix

Use dummy cache backend for tests.

Fallback to default cache if course_structure_cache doesn't exist.

This is primarily to reduce load on MongoDB, where we've lately
had performance problems that we suspect are caused by very
large course structures being evicted from MongoDB's cache. This
may potentially give us a path to better performance as well,
but that's not the goal of this commit.

Surprisingly, LZ4 seemed to actually run more slowly than zlib
for this. Possibly because of some overhead in the Python
bindings? GZip was also surprisingly slow given that it uses
zlib underneath (something like 5x slower).

Use separate cache backend for caching structures.

Abstract out course structure cache.

add datadog metrics for compressed course structure sizes

Since we're using a different cache background, we don't need to have a cache prefix

Use dummy cache backend for tests.

Fallback to default cache if course_structure_cache doesn't exist.

This makes these files importable, and puts them into the new best guess
as to where files should live.

This is compatible with Django > 1.5, and allows us to re-enable the ProfilingDebugPanel

TNL-1380

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.