STUD-1629

LMS-2807

There is a bug I need to fix when a student re-visits a problem that they
had solved previously without masking enabled, their old state does not
load correclty.

It's simplest to turn off masking now, and re-enable it with the whole fix.

The change here is a small deletion to remove the masking=True logic, and more
lengthy changes to adjust the tests.

The bug is with the case of multiple-questions with targeted
feedback within a single problem.

Refactor the targeted-feedback tests to house
the most common xml in external files.

Move the capa test load_fixture function to __init__.py so it
can be used generally.

Instead of refreshing only the necessary parts,
"Run Code" reloads the whole page.

This commit fixes this behaviour, by refreshing the
specific elements belonging to the Problem instead.

Also tidies up indentation.

This commit updates miscellaneous files in common.

These keys are now objects with a limited interface, and the particular
internal representation is managed by the data storage layer (the
modulestore).

For the LMS, there should be no outward-facing changes to the system.
The keys are, for now, a change to internal representation only. For
Studio, the new serialized form of the keys is used in urls, to allow
for further migration in the future.

Co-Author: Andy Armstrong <andya@edx.org>
Co-Author: Christina Roberts <christina@edx.org>
Co-Author: David Baumgold <db@edx.org>
Co-Author: Diana Huang <dkh@edx.org>
Co-Author: Don Mitchell <dmitchell@edx.org>
Co-Author: Julia Hansbrough <julia@edx.org>
Co-Author: Nimisha Asthagiri <nasthagiri@edx.org>
Co-Author: Sarina Canelake <sarina@edx.org>

[LMS-2370]

fix system->capa_system in capa_problem change for anonid

Features by Jeff Ericson and Nick Parlante squashed
down to one commit.

-shuffle of choices within a choicegroup (Nick)
-answer-pool subsetting within a choicegroup (Jeff)
-masking of choice names within a choicegroup, (Nick)
 used by shuffle and answer-pool
-targeted-feedback within a multiplechoiceresponse (Jeff)
-delay-between-submissions capa feature (Jeff)

Covers external grading and certificates.