feat: in-context discussion for units can be disabled by default

[kaustavb12]

Description

Currently for all new units which are added to a course, the discussions sidebar is enabled by default, which the course instructor can disable individually.

This PR, adds to the ability to disable discussions sidebar by default. That way all new units which are added will have the discussions sidebar disabled and can be individually enabled by the course instrutors.

Upstream PR : openedx#35414

Testing instructions

Part 1: Discussion Sidebar disable by default

1. Check this sandbox has the correct edx-platform branch and has the UNIT_DISCUSSION_SIDEBAR_ENABLED_BY_DEFAULT flag set to false.
2. Login to the sandbox https://lms.oc.eshe.opencraft.hosting/lms-login
3. Go to Studio
4. Create a new course, add a new unit and publish
5. Wait for a minute and then check the live version in LMS
6. Make sure the discussion sidebar is missing for this unit.
7. In Studio enable discussion for this unit and wait for a minute.
8. Go to LMS again and check that the discussions sidebar is now available.

Part 2: Discussion Sidebar enabled by default

1. Check this sandbox has the correct edx-platform branch.
2. Login to the sandbox https://lms.kaustav-two.eshe.opencraft.hosting/lms-login
3. Go to Studio
4. Create a new course, add a new unit and publish
5. Wait for a minute and then check the live version in LMS
6. Make sure the discussion sidebar is available for this unit.
7. In Studio disable discussion for this unit and wait for a minute.
8. Go to LMS again and check that the discussions sidebar is now available.

Deadline

"None" if there's no rush, or provide a specific date or event (and reason) if there is one.

Other information

Private Ref : BB-9112

[xitij2000]

Looks good, would just like the name to be changed.