This adds a new django app -- edx-when -- that will copy start and due dates to mysql and allow per-learner overrides in the instructor dashboard, using the existing IDDE interface.
It adds a data migration for existing IDDE data.

* INCR-185 Modernize common/lib/xmodule/xmodule/assetstore and contentstore

* Adding check for python version when making integer contracts

Revert "Revert "REVMI-62 Return gated blocks in the course blocks API response with an authorization denial reason""

Revert "REVMI-62 Return gated blocks in the course blocks API response with an authorization denial reason"

Co-Authored-By: davestgermain <dave@st.germa.in>

Signed-off-by: cclauss <cclauss@me.com>

In Studio: Introduces a new option, `after some number of attempts`
and a new entry box for specifying the number of attempts.
This allows course creators to specify that a given question's
answer is only viewable, i.e its show answer button is visible,
after the learner has attempted answering the question -
by hitting the submit button - a given number of times. Included
in this commit are unit tests for the new feature.

Update version of edx-proctoring and update copy to refer to onboarding exams in Studio for proctoring backends that support onboarding exams.

This PR is based on #19284 and is part of the
series of work related to the proposal #18134.

This PR avoids the assignment of
anonymous/unenrolled users to any cohort when
course is public. Anonymous or unenrolled users
will only see content that does not have a
content group assigned.
The "View Course" link to the course outline
is shown on the course about page for a course
marked public/public outline.
It also makes course handouts available for
public courses (not for public_outline).
This PR also hides the different warnings and
messages asking the user to sign-in and enroll
in the course, when the course is marked public.
It modifies the default public_view text to
include the component display_name when
unenrolled access is not available.

When an unknown content type is encountered, it's imported as a
RawDescriptor, which will preserve the OLX and export it back out. But
if we import a course while an XBlock is installed and then export it
after that XBlock is removed, we export RawDescriptors that never got to
save the original OLX and have a blank "data" field. Attempting to
export this used to fail and break export altogether. We now test that
the export continues to complete, and just skips over anything it can't
serialize out.

Note that this will stil export pointers in the export, so if you
uninstalled a "AmazingBlock" and exported, you might see something like
the following in a vertical's XML::

<vertical display_name="Unit">
  <amazing url_name="2edebb68d5734395a06b8a62b9bb677e"/>
</vertical>

However there would be no corresponding file at:
  /amazing/2edebb68d5734395a06b8a62b9bb677e.xml

In fact, there would be no /amazing directory at all in the export.

The better long term solution is probably to leave the pointer as-is
and export some generic file that can't be mistaken for OLX (say a
JSON file) that represents the raw key-value data we have in
Modulstore for the now unknown XBlock type. However, this commit at
least keeps export from crashing out entirely.

change title attribute to aria-label on Play button for better screen reader UX