Feanil/more bokchoy fixes

The underlying data is put into json documents that expect unicode
strings not byte strings.  In python2 lxml provided strings that were
auto coerced but in python3 it provides byte strings.

In python2 there would be some auto coercion done so that the orginal
code here works but that's not the case in python 3.

Remove extraneous pipes

BOM-366

changes made as suggested

minor change

changes made as suggested

test fixes for python 2

removed debug statement

quality fixes

quality fix

quality fix

BOM-475

BOM-420

BOM-479

BOM-435

Fixing python3

Blockstore opaque keys moved to external repo

BOM-478

BOM-477

... on proctored exam attempt deletion

JIRA:EDUCATOR-4642

Drop RUN_AS_ANALYTICS_SERVER_ENABLED

fixed all file tests

This is an ancient feature flag that uses an archived and
unsupported support library (edinsights, edx/insights).

DEPR-44

Fixing python3

Fixing python3

BOM-405

python3 compatibility

python3 compatibility

user agent regex updated in mobile platform

BOM-446

BOM-412

PR #21614 made this upgrade, but it was accidentally
undone by a later PR that merged an out-of-date
'make upgrade'.

EDUCATOR-4635

This is the third in a series of commits to create
a Python API for the LMS program_enrollments app.
It does the following:
* Creates api/ folder.
* Moves link_program_enrollments.py to api/linking.py
* Creates api/reading.py for enrollment-fetching
  functions.
* Updates rest of app to use api/reading.py when
  it was going directly through the models before.
* Other misc. cleanup (isorting, unicode_literals,
  line breaks, etc).

Still to do:
* Create api/writing.py and update app to use it instead
  of going directly through models.
* Create api/reset.py and api/expire.py, which the management
  commands call out to.

EDUCATOR-4321

This is similar to use_read_replica_if_available(),
but it just returns either 'read_replica' or
'default', for use with the Django ORM using()
method. Syntactically, it fits nicer with queryset
method-call chains.

BOM-472

BOM-411