Make XModule use the same get_children that XModuleDescriptor and XBlock do, and make it returned the same CombinedSystem

Prior to this change, if a CachingDescriptorSystem was prepopulated
with XBlock field data (`module_data`), then it would be used as
the DescriptorSystem for blocks loaded through it. On the other hand,
if it had to fetch new data from the modulestore, then a new
CachingDescriptorSystem would be created by the modulestore, and used
by the newly returned object.

This commit allows the modulestore get_item and get_items methods
to accept an existing CachingDescriptorSystem to augment with data
from the backing mongo store, and to use as the DescriptorSystem
for newly loaded XBlocks.

Remove unused imports

Fix test_instructor_dashboard.py tests

Implement full preferences API

TNL-1493

See https://openedx.atlassian.net/wiki/display/TNL/User+API for details

This file was never run, because of a missing __init__.py.  It needs a
super() call to make the ModuleStoreTestCase work properly.

Then one test failed, because of a missed change from dollar to
configurable currency.

Update the bok-choy version being used. TE-783

Simple capitalization fix

Remove old env settings files

pass opts From PythonTestSuite to NoseTestSuite

Instead of referencing the example externally, we now explain the
setting inline.

This contains hard-coded reference to out-dated courses.

This looks to have been a personal configuration file.

Is this used? It contains MITx references.

These all date back to 2012 and are hard-coded for specific course runs.

Define TENDER_SUBDOMAIN so that we can still use vanity URLs

Update translations (autogenerated message)

Revert "To support UNICODE xblock.location"

Lms should be independent of mathjax

Unit page doesn't load when mathjax is down

Add Mocks to Enrollment API Doc Config

To support UNICODE xblock.location

When mathjax is down studio page doesn't work as
it wait for mathjax to load. I replaced the hard
dependency with soft dependency now unit page can
load with mathjax.

When cdn is down specially in the country like china
and others user face problem as application doesn't
work as intended. Instead of making mathjax hard
dependecy I have converted it into soft dependey
so that it won't cause user problem to use application
smothly if mathjax is not present.

TNL-1498

Fix Pylint violations (medium)